Well, comparing it to a SR20DET is not a valid comparison. Consider that both the EJ20 and 4G63T also run Mitsu and Garret turbos, but also run MUCH higher standard boost pressures than the SR20DET. They make just as much, if not more hp than the SR depending on how much boost you throw at them. And both of those engines have been reknowned for their ability to take boost. The S14's turbo runs out of gas at around 13-15psi... a 16G turbo will push past 20psi and the stock internals of the 4G63T can take it. Max effective psi on the S15 is a tad higher at 17psi.
They're made/tuned differently... plain and simple... now in regards to the Saab Arc/Vector getting a TD04 housing turbo (heck, there are quite a few TD04s) if you can turn up the boost pressure with a chip or simple manual boost controller, it should help a lot. It really depends on which TD04 Mitsu turbo is in there... and it really depends on the strength of the factory internals and fuel to determine how friggin fast you can go after that.
All that being said... the Saabs weigh how much? Silvias weigh what?