The diameter of the muffler and associated tubing will only have an effect on the max hp you will achieve at top revs. Maximum torque is pretty much set by the camshafts and fuel type (with upgraded ECU), and being in the mid range of engine revs will not be affected by exhaust diameter/flow rates.

Summit sells hi-flow cats for like $70 i think. I agree on having at least one small Cat. It gives the exhaust a smoother tone on our rigs.. they sound kinda rough without them. I had gutted both Cats once and it was loud as hell with my 3a ricer muffler. Then I put a new OE pre-cat on it it sounded much better and quieter. Then decided I didn't like the ricer sound and went to Mach1 Muffler and they replaced my stock muffler with a flowmaster knockoff. It has no label on it so I don't know what it is. But it looks just like a flowie.. The sound is perfect in my opinion. Same sound it has in my videos. Alot of the noise my kia put out in the vids was coming from my ram air intake while I was 50% or more throttle.

Bestmufflers has their Kia Sportage headers up for $355.00 now.
Best Mufflers

Kia Sportage 1997 to 1999 2.0ltr 4wd Wildcat Headers [WILD256]

I originaly went with 2 1/4" cat back with a MagnaFlow muffler and had a noticeable power increase. I ripped a hole in the MagnaFlow at Moab and replaced it with a Summit turbo, power increase gone. I think the MagnaFlow made the diferance, not the bigger exhaust. I went to 2 1/2" because I am planning a turbo install down the road. I have a 2 1/2" MagnaFlow cat going into a round MagnaFlow muffler in front of an ovel MagnaFlow. My power is good but it is very loud but I am banking on the turbo to quiet it down. With our little 4 poppers I think a good flowing muffler makes a bigger diferance the pipe size over stock. I like MagnaFlows for thier ability to free up power but on a FE3 motor they are loud.

Dennis

Very tempting. Too bad they don't make them for 2000-2002 so you don't have to modify the exhaust.

I do believe the straight through design of the Magnaflow's is what makes them a much better muffler than Flowmasters or turbo mufflers.

I went with a turbo because I was afraid a Magnaflow would have been too loud.

I belive it has to do with the precat and cat situation and the existance ( if I am not mistaken ) of 2 O2 sensors, and not whether it bolts on the block or not...

There is a before and after oxygen sensor setup in my 01 sportage. Without the pre-cat, my chk eng. light would stay on always, but this doesn't hurt the drivability. It still has closed loop data from the primary 02 sensor. the 2nd one is just to check that the cat works. You CAN gut the 2nd cat without it interfering with the ECU so long as you leave the primary. I hear there's a way to make a secondary sensor fooled without having any cats. There's even plug ins for 'race applications only' that go into the wire harness that would otherwise plug into the o2 sensor. I wouldn't let the sensors stop me from buying something like this.
Being that our aftermarket resources are far and few between, I think that we should post the one's we know of on a sticky with categories: engine perf, lifts, body, fan sites, etc. i dunno.. random idea.

So to sum up, if I'm right...

Extractors, stock exhaust pipe, straight through muffler.

Is it worth the money?

well it finally dried up enough for me to check it and it's still the stock 2" <img src="/forums/images/graemlins/rolleyes.gif" alt="" /> ...was gonna try to upgrade but came across a killer deal on some wheels..guy says they are solid centerline 15x8 wheels but they could be american racing outlaws i dont know how to tell the difference...there went my upgarde money so just a cheap replacement muffler it'll be for now