it's a litle late but better late then never
I think that you could use the rear axel hubs from a mk 3 GTI or VR6, they should fit. at least, 16v rear brakes etc are freely exchangable within th mk1, mk2 and mk 3 versions......
For the front you face various problems, what kind of enigine do you have at the moment? because the throtle shafts have various thiknes, so if you whant to convert your hubs from 4X100 to 5X100 you need to measur a lot of parts, joints, steeringrods etc etc, and also the attachement of the front suspension to the hub ( !!! )
but why the %^&%%@# do you whant to covert your hubs to 5X100?????
if its brakepower your looking for, just convert the rear brakes to disc brakes of any 16v, and buy adapters for mounting G60 brakecalipers on the front. this would be much more cost effecient, much easyer and an equal result brake prefomance. ( if you whant to take it a step further also replace the master brakecilinder for a G60 version )
good luck!