I really wish people would stop saying things in posts in which they clearly don't understand.
There is no maximum to what any engine can achieve, it's just a case of how big your wallet is...
The SR20 is capable of making at least 800 HP...this has been done numerous times in the S14 200sx...albeit a shit load of money thrown at it.
If you want big power, forget NA unless you want a deafening engine, that's a pain in the arse to drive. Also, to get over say 200 HP, you better get saving, because it gets expensive. Unfortuanely the cylinder head design isn't as good as it should be, especially in comparison to the Honda B' series engines.
If you have a budget of say £1000...then get the usuals....manifold, filter (housed), exhuast, ECU...etc.
If you can stretch to £3000, then get nitrous. The stock internal will easily take a straight shot of 100 HP!
If you can stretch to £5000...then go turbo. This should be more than enough to either buy a GTiR swap or a very successful turbo conversion. There are many turbo kits available from example...Hotshot, JGY, JWT, Powertech, FMAX...etc.
And don't worry about engine strength...the SR20DE is a strong mofo, and is easily capable of taking 500 crank HP in stock internals.
Cheers
Dave
P.S. You may get gearbox issues above 300 HP...just like the GTiR!!!