i have the following done to my 99 sse (k&n cold air intake, , msd coil packs, msd 8.5mm superconductor wires , dimondfire plugs,180 thurmostat, muffler back exhaust, zzp hv3 , zzp pcm tune,zzp throttle body,ER rockers,Ls6 valve springs, zzp pl) would i gain alot from a downpipe or should i just replace my cat with a free flowing one ?

What is a muffler back exhaust?

new from the muffler back its often called an axle back exhaust.

That'* what I was thinking. Cat back is everything behind the cat, so when you said muffler back I was thinking you put new tips on.

Our downpipes don't have as much of a restriction as say a W body. The biggest restriction is the front exhaust manifold. A Plog may be a better investment if you are looking to free up the exhaust.

i already have a power log , and thought a high flow cat would help?

It might, but don't expect anything noticeable from it...except a slightly louder exhaust.