Regarding the exhaust -
There are plenty of good online resources regarding diameter requirements for hp rating. If you're going to exceed 300 hp you will need to go with a dual 2.5" system, and 3" if you get close to 400, which is larger than the 987 collector (2.25"). (The segment of tube immediately following the second cat on my 2.7 was only 1.5" diameter! What the hell stuttgart?!?! That's so small that I can't even find a HP rating for it!)
There are two options here - use aftermarket catless headers and fabricate a very short 2 to 2.5" flange adapter to the collector and then run 2.5” from there (this is what I am in the process or finishing up now and will post a diy on shortly) or to have custom headers made. The second is more challenging, but you can cut the flanges off your stock header and create a shorty system in a fairly straightforward fashion. The guy who welds for me estimated it would take 4 hrs to do both sides. If you want I can elaborate on the design we were considering.
In terms of low end vs high end torque, long tube vs short tube, it's all about flow and heat. Keep the flow smooth and keep all the heat in and you will get the best results across the board. Help her breathe, that's what this is all about.
And please for the love of all that's holy, don't bring up exhaust back pressure! This forum is far too dignified to discuss such silliness
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