no offence but if i was you i'd sell on the 8 valve and either buy a 16v or better still a vr6 if you still want a mk3, or go back to a mk2 like fish&chip boy said! for the amount of extra power you'd get out of an 8 valve you might as well put the extra money it'd cost towards a vr6.
its just a power to weight thing that makes the mk3 slow. unfortunately they are heavy old cars.....