I've spent more money trying to save money than the value of my boat, lol. Hopefully you've solved the problem though.

I just put these in and was shocked at how well they quieted things down although I haven't given them a running test yet.

I think the GGB is my only option at this point. I am looking into them now trying to figure out which size to get.

I thought about getting the 10" long units but was surprised at how much the 5.5" units mellowed the bark. It has only been on just the hose so far and don't know if they will cause any performance loss or troubles like you're having yet. I'm running CMI headers on a 502 with Hardin tips.

Are your tails dry?

My water dumps are at the end of the tailpipes, I can't see a way that these inserts could cause any issues, at least not with water. Maybe some back pressure....

how do these things lock in place, allen set screws ?

Bolt thru tailpipe. Ie: drill hole thru tailpipe, drop bolt thru lower tab on insert and thru drilled hole in tailpipe. Install nut.

My headers drop the water in at the very tip about 8" before the transom. My Hardin tips already had a hole in them so it was literally a two minute installation and two minutes to pull them out if I want. They didn't interfere with the Salisbury flap at all.

My tails go through transom about 4 inches and the water exits at the very tip

GGB makes these (slotted) for that type of set up.
http://marineexhaust.ggb.ca/insert/slotted-insert

Good articles with info from several builders we know here on OSO:

http://marineexhaust.ggb.ca/1300hp

http://marineexhaust.ggb.ca/dynotests