welcome
thats quite the upgrade, performance wise anyhow. I'd look into getting a lid for the air cleaner assembly. A while back GM high tech performance tested a bunch and SLP made one of the better ones. I think TSP made the other good one, but dont quote me. A cat back would be next on the list. After that, I'd either get headers and a Y-pipe (either catted or off-road) or maybe an LS6 or FAST LSX manifold. Tuning is very important and luckily for you, we have a great LS1 tuner in the group, foff667. thats pretty much all the budget stuff I can think of, even though the FAST manifold costs a decent chunck of change.
I'm sure the guys who actually own LS1 cars will chime in soon, then you can get a better idea of what everyone else is doing with their cars.