Put Slp 2s on that setup... Slp1 with and or x AND long tubes..?!! Its going to sound like sh*t and way too loud. I know from experience. The only slp setup i recomend is slp1 catback on a stock setup.
But if you want my personal opinion. I wouldnt even go with the slps at all. Your car would sound soo good with some magnaflow or borla mufflers, they will tame it a bit and give it that nice race sound.
No prob man. I know how it is. Racked my brain for months about that with my 99 gt. I used to like flowmaster but meh i don think they are that great anymore.
As for your other question, if i were you, i'd call BAMA and ask them what would be the best combo for gains for whatever your setup is... I've gotten some decent gains outta bbk cai with 70mm throttle body.(gt). I use a k & n cai on my cobra. Bought the car like that tho, i dunno if they are consistant with what they are rated.
I think there are better combos out there. I'm sure there are other forumz about this. Good luck man.