I was suprised to see the gain myself but this is on twin 496 mags.
when I did the switch on my 290 with twin 454s other than a louder exhuast I did not notice anything else.

I went from open exhaust to the 6" version of the Corsa mufflers on my boat last year. I noticed no change at all except I like the sound level much, much better now. Call me old I guess. Corsa told me they would cause no issues up to the 600HP range.

I had changed them on my old 23 and I'll say they was defiantly a change in the sound. It was much louder. Loud enough that the wife really noticed and of course I got what did you do????? Speed wise no difference but the 23 was a real tank. I think your right the bigger HP's motors may see some increase.

verbi69 what kind of speed gains did u see from the raylar kit i have been considering the kit not sure if i want to spend that kind of money or just step up to twins
thanks john

Hi John,
My boat would run a steady 64 mph with a labbed Bravo 1 26 pitch and with the Raylar now I am seeing 70 mph on gps.
I have not run the boat with the labbed prop because I had taken it off and sent to to BBlades for some cleanup and rework.....so I expect to see another couple mph for sure.
I am also running the stock exhaust - no headers yet.
The kit is not cheap for sure and I had a professional shop do the install along with a full tune up like plugs wires all filters belts and impeller and drive service so the bill added up quickly.
I love my boat and wasnt ready for twins so there is no replacement for power....and this thing really works now.....there is no lack of power anywhere in the rpm range...it pulls hard.