i'm runnign 280's and done have locking problems, as said tho if you just stamp on the brakes they'll lock the wheels instantly, but feed them in you can feel the arse end of the car lifting somethign spectial and it stops pritty damb quick.
tho have had grabby brakes before, try chaning the pads , some cheep pads have some quite nasty tendancies so lob them in preferance for ebc ferdoo or some other good brand and them change the brake fluid, if that dosent cure it have the suspention and trackign / camber looked at