Starting to think about the spec for a new bike build.
When running different tyre widths f/r does anyone run different rim widths?
Thinking possibly of 27.5 x 3 on i45 rim front and 27.5 x 2.8 on i35 rim rear. The i35 may have to change as have not found any for sale yet. I think all the new B+ bikes with boost rear ends are using it.
Dropping to an i25 rim will give a narrower profile to the 2.8 tyre which is probably better for chain line clearance with 2 x 11 gearing.
Any thoughts or experience of this?

No experience of 25mm rims with 2.8 B+, but on WTB I45s I thought that there was a fairly small range of pressure that made them feel good. Too low was squirmy and lacked cornering stability to match the grip potential and also would pinch fairly easily if you mis-timed something, too high (~20psi) and I felt there was no advantage to my P35 (30mm internal) + Ardent 2.4 usual combo, in fact I preferred the larger 29 wheel to the higher-pressure B+ once pressures and contact patch areas were approx the same. For me B+ needs to use lower pressures to be worthwhile and narrow rims go against that. A 25mm internal rim would feel about right on a 2.2-2.3 but for the pressures I found good for B+ I suspect it'd undermine the tyre too much. I'd aim for 35mm internal as a minimum, 45mm ideally.