What horsepower differences do you get goons from stainless marine manifolds to cmi headers

The differences will come from the horsepower, top rpms and build of your engine, the model of CMI or header you are talking about and the exhaust routing after the header out of the boat.
Answer some of these and we can try to give you a good estimate.

Best Regards,
Ray @ Raylar

What Ray said. On a stock 496 or 454 you will most likely see zero speed gain despite the claims made. Ask me how I know .
On a high performance engine like the 525 EFI, headers are an important part of the package.

I saw no difference when I put emi thunders on my 502, and it has a larger cam, larger intake, and bigger carb..

But they do look and sound cool!

I saw zero gain in performance going from Gil's to CMI's on a hopped up 525SC.

I wouldn't expect ANY speed gain going from stainless marine manifolds to headers unless you buy the very best CM'I's made and even then if you saw a mph I would be amazed. You have already got 90% of whatever gain is going to be had by going from stock exhaust to the Stainless marines. Stanless marine manifolds have been proven to run same speeds as headers in most circumstances, don't under estimate them!

What about stock mercruisers to cmi etops?

Thats a good question, they claim 25-40plus hp over stock manifolds , about 10 or 11 years ago I was running a blown 502 with 7 psi of boost, was still using stock merc iron manifolds, boat ran mid 70's. After 2000$ and a weekend of bloody knuckles, searching for 4" hoses, etc, I had installed a set of brand new lightning headers because they were supposed to be good for 400-600 rpm's over stock on a blown motor. Well, boat had a tiny more mid range punch but picked up ZERO top end, tried differents props, made zip difference, I couldn't believe it! I am sure that at a certain amount of cubic inches and or boost that stock manifolds would just kill the power of a engine but you would be surprised on a un-modified engine how little difference it really makes, I think 1/2 the gain is from the weight loss on stock apps!

unless u have a big inch,supercharged engine,u would be throwing money away.from a performance standpoint ,but the bling effect from those cmi,s is a different story.