Newbie Question

[FishThis]

Hey guys just wanted to say thanks for all the info I have gotten from this site. I joined a while ago when I bought an 84 V that needed some TLC and since then I have gotten married and bought a house so needless to say the V had to take a back seat for a while. But slowly I am trying to get it right.
Which brings me to my question. I went through an issue with some bad gas and ended up adding a Racor with water seperator and some awsome fuel treatment (STARTRON). Also ended up changing a fuel fitting that was clogged up. When I got the boat it would top out right at 40 MPH and know I am only getting about 26 MPH out of it. It has a 175 Mariner on it. Kinda a side not that I dont think matters but I took out the leaking internal tank and have been running off an external tank. I have put new fuel hoses and new plugs in as well as flushed the carbs with fresh gas by removing the drains from the top down as i was pumping it through. I checked compression a few minutes ago and all the cylinders were around 75. One thing that was weird is when I opened the throttle before i did the compression test it let out a hiss sound. Could something be causeing a vaccum. That would definatly cause loss of power.
I know thats alot at one time but i have used the search function extensively on this site but this problem is stumpping me.
Thanks in advance
MATT