Can anyone tell me the difference in suspension coils from a BF GT-P to a BA XT?
I know there are differences but what are they? (height / ride / etc)

my mate just did the exact thing with his ba xr8. got his hands on some gtp springs and shocks (100km old) and fitted them to his xr8. we started at the rear as the front already sits ok. it made no difference. maybe 5mm but that could just be tolerance with spring manufacture. spring rate seems the same also. if you are putting them in an xt expect it to drop down to around xr height which is 375mm centre of hub to guard. fronts i would not touch as they will def have a different rate due to the fact there is normally a boss 290 sitting in between them. prolly act like a lift kit on the xt and be stiff as buggery.