i'm going to pee on Crezz's chips here, in every day driving the 8v is just the same as a 16v (I've owned both) except its more reliable and more economical and 5 insurance groups lower!
Average 8v puts about 125bhp out despite what book figures suggest, whereas a 16v is about 155bhp. 8v's can push out a lot of bhp, theres a 220bhp one on here thats running a nitrous mix. Supercharging with an Eaton or similar would see around 170-180bhp.
If your going to do any major engine work new engine management is the way to go for sure as it will make the best from it, however custom code, jabba and revo do new chips for the existing ECU to suit new components.
If you want as much power as you can get out of the box and can live with a screaming engine and a less comfortable drive, get the 16v it won't dissapoint. If your using it as a daily and want something that will be more reliable and nearly as good with room for improvement go for an 8v.
There are quite a few on here that have NA engine mods, so search for these users:
Rocket Ronnie (nitrous)
F2Stu (track slag)
jcs356 (track slag)
Crezz (not really modded but he knows about 16v heart ache)
Dinky Dave (i think this is a 16v with cams)
m0rk (he probably has the best collective MK2 knowledge)
Cupra Kid (i know about 16v heartache)
No doubt i've missed some.