Mk3 GTi is completely different than the non GTi's. 2 litre for a start.
As for Mk3 1800 injection and Mk2 GTi there is quite a big difference - all the control unit is different, the fuel pump pressure is different and all the inlet manifold and throttle body is different. As far as I can work out, the single point injection is effectively a carb with an injector sat in the middle of it.
I'm not entirely sure what the best way of making the 1800 go better is, I did look into it a while ago for a mate of mine who used to road rally a Mk3 1800 Driver and in the end we decided that there wasn't a great deal that could be done without spending a shed load of cash.