The best way to reduce the air gap and gain efficiency is to move the stator plates closer to the 12.3mm rotor rather than using a bigger rotor... I don't think the motor manufacturers make the stators like that though, so you'll have to play with rotor sizes intead, but I think the 12.2mm to 12.3mm rotors are the best choices out there....
In addition, with the high rpm of a 2.5t motor, the closer the mass of the rotor is to its centerline, the better... The further it is, the more the rotor will fight itself and explode inside.... In high rpm situations, smaller rotor is always better !!!