Has anyone that has had mufflers on their boat removed them and noticed a H.P. increase or a speed increase. I'm thinking about taking mine (Gattlins) off but I don't want to do it if it is just going to make it louder and not gain anything. The manufacturer says they don't rob the boat of any H.P. but its just seems like they would have to create some kind of back presure.

You will not gain much other then the sweet sound! Usually you gain 0-10 hp tops. Generally you gain nothing you can really feel or see on the speedo. You may feel a little more mid range and possible 1-2 mph top end, maybe. You could even gain a little rpm on top end, maybe 50 to 100 rpm’s tops. In most cases you don’t really gain anything, but you get to hear that beast roar!
Johnny

My motor makes around 950 hp/ft lbs tq,I'm running stainless marine hi tq3's with 5" tails and ggb's,my boat runs NO faster without mufflers and idles like crap so I always run them. It gives you a headache and brings un-wanted attention from johhny law with them off too,Smitty

I run CMI Sport Tubes with a 4 to 4.5 ID stepped tails and CMI Aft-Fire mufflers. At 6500 RPM @ 100 MPH I lose only 1 MPH in back to back runs. This is with the mufflers installed and in the open position vs. removed. The tails are dry with the water at the TP exit.
Pretty reasonable for the piece of mind at idle and at low speeds.