Don't know about Stainless's specs, but EMI drops weight by about 60-70 lbs on a 350, and claim approx 10% hp increase. I assume Stainless is about the same specs. I replaced my stock Mercs mainfolds on a past boat with a 454, dropped around 70 lbs. The boat planed off a bit easier, and probably picked up 1-2 mph, and 50-150 rpm's depending on conditions (water conditions, temp, how many on board, etc).
I'd say that if your manifolds are due, definitely look at Stainless, EMI, etc. But if this is strictly a performance upgrade w/o adding anything else (aka blower which requires more exhaust flow), then the $$ invested may not equal a sizable return, especially if you can't do the job yourself and are paying for shop labor rates. I only did mine because they needed replacement. Hope this helps.