I went for all this, basically, because the opportunity presented itself and the argument about the flatter trajectory of the .177 finally convinced me. With the roller-coaster trajectory of the .22 at 12ftlb there's the constant difficulty of trying to estimate where the pellet will be in its arc when it reaches its target point.
If you zero at 35 yards with a .22 there's a variation over, say, 40 yards of about three inches up and down through an imaginary line from the end of the scope to the target. The pellet starts two inches below this line, climbs up to about an inch above it and then immediately after 35 yards drops below it again in its downwards fall. So there's a three-inch margin for error depending on where the target is and constant adjustments up and down need to be made in the reticule to accommodate this while aiming. (The graphs below are from ChairGun Pro available free here.)
With the .177 at 12ftlbs however, the arc of the lighter pellet will stay within one inch from about 10 right the way out to about 40 yards (.177 RWS Superdomes weight only 8.3 grains against the 14.66 grains of .22 H&N Field Targets). So in theory that means that if you can judge that the rabbit is at a distance anywhere between 10 and 40 yards then you can be fairly confident that a shot at the intersection of the scope's cross-hairs will result in a pellet hitting the target area. If you compare the length of the different pink kill zones on the two graphs you can see that they do bear this out.
My distance estimation is not so great but even I can tell if a rabbit is between 10 and 40 yards; so purely on the basis of this there's a much stronger case for going with a .177 for fieldwork.
To get the pellets out at anything up close to 12ftlb a .177 needs a spring with a few more coils on it than one in a .22 - and you can feel the greater effort needed to cock the rifle now. (The rifle was of course chronographed before leaving the shop and it was doing a healthy and legal 11.78ftlbs with the RWS Superdomes I'm using.) I've heard it said from people commenting here that with this stronger spring the increased release vibration in a .177 would act against any gain that the flatter trajectory could provide. This did seem to make sense to me but (and I can only speak about my rifle, of course) thanks to Tony at Sandwell Field Sports, if anything the gun now fires with perceptibly less spring judder than it did before the work was done.
My damn cold is still hanging around so I suspect I won't be going out today to see if all this theory on paper translates into a greater incidence of rabbit on the table - but I'm hopeful for the future.