exhaust manifolds, what do I have?
 

This exhaust manifold is what was on my 99 2.5l when I purchased it. Are these stock? Is there normally a catalytic converter on these?
http://986forum.com/forums/uploads01...1385581800.jpg

The secondary "pre" catalytic converter didn't appear until 2000. That looks like a stock 97-99 manifold.

Steve

aftermarket exhaust manifold
 

Would the aftermarket header exhaust manifold really make any improvement on power since this doesn't have the pre-cat?

Small improvement at high rpm, probably noticable loss at low rpms.

If I had a 2.5 I would have stock manifolds ceramic coated inside & out to retain heat inside rather than dissipate that heat to the heads & camcover area.

May be able to gain some flow by grinding welds at tube entrances & exit also.

No matter what you have for headers, those 400-600 cell cats are going to remain very restrictive.