Without the CAT you might have gained 2-3 hp, but probly no gains , the OEM Cat is pretty high flowing.
Dual exhaust
might possibly give you 8-15 HP.
Your Cutlass has either a 350 or 400 turbo trans, the 400 turbo is better for power adder, they can handle 250+HP & ft lbs of torqu ...
easily.
Just dont do neutral drops!
If you were putting out 350+ hp then you could rebuild the trans with some performance clutches and a shift kit.
A shift kit can usualy be added with out removing the trans, just dropping the pan & valve body.
Hope that helps Good Luck