[Batman]

Here is the order you should go:
Lid/filter (they are all good but SLP, TSP and whisper fit the best) $150
smooth bellows $30-$45
Port the T/B $150
High flow Y-pipe (NOT SLP!!! It fits like crap!!) $250ish
Cat back $200-$600
Then buy a Z06 MAF ($179) and get the PCM re-flashed by a pro ( I HIGHLY recommend Rapid Motorsports in birdsboro PA, but expect to pay at least $400+dyno time for any full tune and trust me it is worth every penny), easy 50-60RWH. If you are planning on doing internals/power adder or anything like that get some longtubes instead of the Y-pipe. Stock manifolds flow very well so if you aren't at least doing a cam you may actually see a loss from longtube headers in the low end power range (trust me the LS1 needs all the low-end it can get).

I would suggest Magnaflow for the exhaust, I had the loudmouth and the magnaflow put down more HP and actually HAS a muffler. I only gained 2 HP from cutouts over the magnaflow exhaust. I would highly recommend staying away from SLP, I had a car covered with their stuff and now all I have left is their lid. I was not happy with fit and finish on anything, especially their longtubes and Y-pipe. Their stuff is good, don't get me wrong, but there is better for the price range IMO.