On my EV2, sizing AA Field skirts down to the head size of 4,51mm seems to have little if any effect on velocity or accuracy but on my wifes EV2 the velocity drops about 8-10 ft/sec with sized AA field.
Any changes in group size may be more to do with the change in velocity. Both our EV2's are at their best at 770- 780 ft/sec with JSB/AA Field pellets.
If I raise the velocity to near 800 ft/sec my rifle does throw more than the odd 9 on UKBR22 targets. I first noticed this when shooting a series of 3 competition targets on our 25yd outdoor range on a calm but very hot summers day. The firing point is under corrogated sheeting and the temperature must have been around 30-35C. I chronographed the rifle when I came off the range and velocity had risen about 25 ft/sec.