In the US, base 998's don't have a lot of power, we're talking 115 - 120rwhp at most. The technical reasoning behind the bike's low power delivery comes from the exhaust valve sizes, cam profiles and small diameter (50mm) exhaust headers. Ducati had some issues meeting sound requirements with the larger exhaust valves and headers, so they went with a smaller header and used a cam profile which delivered adequate power for the size of exhaust.
Unfortunately as a consequence, the 998's don't respond very well to aftermarket exhausts. Even the later generation 998R's with their higher lift cam profile, still prefer smaller 52 or 54mm exhausts, rather then the big-booming 57 or 62mm corse exhausts of the period. In Europe, Ducati was quick to release the 998S which had a greater lift and longer duration cam which wound up in the 998/999R of the cross-over generation (03/04) it wasn't until 2005 Ducati re-designed the engine again and made huge changes in the head including larger valves and greater lift cam's. The US got the 998S/R cam's in the Monster S4RS which was quite the treat as it was the last generation of standard testastretta's produced before Ducati moved into making the Testastretta Evo's.
So what should you expect with a base 998 with slip-on's? Not much more then 125rwhp and about 75lb torque. Throw some S/R cam's in there, you'll gain a bit with a full 54mm exhaust. I've seen 998's produce upwards of 130 - 135rwhp and about 80lb torque with the proper cam profiles and full exhausts. However, I was only there for dyno tuning and didn't measure the exhausts, so I can't tell you which ones they were using.
Funny anecdotal story, last weekend I was at a track day testing a friends bike and ran into someone who use to be one of my competitors back when I was racing a 749R. He simply came over, shook my hand and said "your bike was really fast, I was the guy you use to blow past on the straights". Just that pat on the back reminded me of all the 998's I'd blow past on the big long straights with my 749R. All of that to say… the 998's really aren't fast bikes. As I said in your other thread, they can be made competitive, but at a cost… Maybe a used 05/06 999R motor is in your future! LOL