Following up. I've ridden around 1500 miles on road as well as the usual MTB miles since making this change (in the same shoes on all 3 bikes I ride currently) and I'm unlikely to move the cleats forward again to the original spot. I went from a mid-position in some Giro shoes that match up pretty well to Shimano's slots to the rearward holes in the plates positioned withing 3mm of the rear of the shoe slots. 12-15mm further back than before.
I made some notes on my early impressions and they all still apply -
I feel better in technical riding situations as my calves are less stressed, I'm more neutrally balanced on my feet. Bunny-hops, floating over roots etc have become a bit easier and more consistent when tired. It's a closer match to how I used to ride on flats pedals.
I lowered my saddles 4-5mm maybe and this has helped add comfort since my saddle-bar drop is a little less. The cleat change also effectively slackens off my seat angle if the cranks stay the same length (ie on my SS) and this feels good. I've always liked steeper seat angles but the further I ride, the more I like to get back behind the BB to help neutralise my weight on the front.
I've built and ridden good mileages on a new road bike with 170mm cranks (and a revised seat angle to closer match my Jones, this cleat thing and other ideas) - yet my legs / feet are in a very similar position as when I had 175s and a more fwd cleat. The shorter cranks are better for long days spinning along and I don't feel a loss of low-cadence power due to the foot/cleat position.
Power output feels better, or at least more consistent, during a long day but I'm not sure if this is actual or perceived. I think this change has helped 'even out' my pedaling, ie I don't spin up to speed as fast or sprint quite as well, but over a long day I'm more even in my output as I've not burned up as many matches earlier in the ride. It's all minor differences but I'm pretty sure it's there. I'm not a natural spinner/sprinter at all so maybe this set up just suits me better.
Conclusion - my cleats were too far fwd to start with? : ) They were centre-slightly forward in the shoes anyway. Now I'd be tempted to say 'start furthest back and go fwd from there if it feels too odd'.

Good summary James, all of that pretty much applies to me too. Andy (ChickenLegs) and I did the South Downs Double last weekend, and after 195 miles off-road, I didn't feel too bad, I was tired, but no bad aches, or RSI issues from bad setup, and the cleat position really helped saved my calves for hike-a-bike sections.

Something Tony told me about crank length was interesting, he said they did loads of studies and cranks length bares no correlation to power-output, so going from 175 to 170 cranks shouldn't actually affect your power output at all, and moving the cleats back should actually slightly raise your power output as your calves aren't absorbing power from higher up your legs.

Glad it's working out for you.

Correct on power output and crank length, assuming the cranks suit you ergonomically that is. Power is work done over time, so a shorter crank that spins faster with less leverage vs a longer one with more leverage spinning a bit slower, it should all balance out. But I think much depends on your natural pedaling style, terrain etc. I'd be able to spin my SS well along the towpath with 160mm cranks but it'd not help on those 30 rpm stall-near-the-top climbs : )

SDW double, good stuff .. you'll be going well come next June.

SDW Double was great, I want to have another crack at it, try to get sub 24, not sure when, I guess once it's wet, it's not a great route, it was bone dry last weekend. We had a few sketchy moments with some very excited bullocks, but other than that, a great training route.

Makes sense with power is effort over time, I have 175 middleburns now, and I'm pretty short, so they are too long really, I need to step down to some 170s on the new build, whenever that happens.

Have those of you who moved their cleats back and lowered their saddles still using them in that position for long distance rides and are you still getting less fatigued? Just wondering as I am setting up some SPD's tomorrow.

Still not changed mine. However I've been riding a road bike this week on my old road shoes, generally I don't use SPD-SL anymore but the bike out here has them. Hadn't thought about the cleat / foot position during the rides until I saw this post come up. So going back to an old/original road cleat position on a bike I rarely ride didn't feel odd to me this week. Did a lot of climbing today, most of it on one big climb, 5hrs or so, felt ok. I may change them for the next ride to see how it feels.

Yes, my cleat are still set right back, and saddle lower, it's great, less fatigue for sure, especially in calves, which was useful for all the walking on BB200!

I have moved mine back now. I suffered a lot with IT band pain in my right knee on the BB200, which meant I was hobbling around half the route, and eventually unable to ride anything except flat or downhill. (Still bl**dy finished though).

Interestingly, I have just got a new bike (2015 Genesis Croix de Fer). I went for the frame set and chose the bits to build it up. Part of that process was making sure it fitted me OK. My Fortitude has 175mm cranks as originally supplied, but we went for 165mm on the CDF. This was mostly in an effort to avoid toe overlap (this was also why I moved my cleats back). The other thing we looked at closely was saddle height. I have a 29" inside leg and am not the tallest fella in the world. I have always just put the saddle where I thought felt comfortable. This has never really caused me any problems, although I am now beginning to think that I have had my saddle too high for a long time and that this may have been the main cause of the knee problems.

Anyway, when I ride the CDF, I can immediately tell the saddle is lower than I am used to. My first temptation was to put it up to where I would normally have it, but I have resisted, as it was set during the fitting, and I think I should give it a fair chance, and for me to get used to it. Now, instead of my toes pointing downwards on the down stroke, I can keep the foot flat, and even drop my heel a little to gain power when needed. I have ridden 130 miles over the week commuting. Mostly on road, but with a peppering of off road too and feel great. No pain, no aches, just lots of smiles.

Last night, I compared the distance between the top of the saddle and the centre of the pedal axle when the crank was at it's lowest point when in the line with the seat tube (to give a straight line). The distance on the CDF is 2cm shorter than the Fortitude. I have now moved the Fortitude to match and will see how it feels when I ride it next.

It would seem there is lots to consider!