The WCF "shorty" headers, The Sprint manifolds from the Fiero store, The KFG headers, and the ported stock manifolds will get you about 5hp (+/- 2hp) gains. The WCF full headers and FOCOA's will give you HP gains, but you will lose low end torque on a stock 2.8L.
Cost vs. Gains, your best choice is to port the stock manifolds. Buying the others will not give you any more gains, But they are built better, look better, and you will have less problems with cracking over time.
Do a seacrh in the archives for "1FST2M6" and "Exhaust". Travis dyno'd many different exhaust set-ups and can offer better details.
------------------
Happiness isn't around the corner...
Happiness IS the corner.