With over 22 years in RC boats I would like to add to this discussion.

1. If and engine's exhaust timing is not at least 160 degrees or more a pipe with nitro of 25% or more will not do much of anything but make needle settings hard to hold.

2. If the exhaust port timing is changed to anything beyond the 160-170 degree range THINGS WILL PICK UP IMMEDIATELY with a tuned pipe BUT intake timing may need to be addressed also. Sometimes a larger carb can help some.

3. We NEVER put a smaller pipe on a larger engine. It is just not a practice that will get you much with higher nitro. We DID put larger pipes on smaller engines and set them at a longer distance to center of pipe. It will however effect needle settings.
4. Check the timing first , if it is below 160, get that changed, use 25%nitro, and a pipe setting of around 12 inches and work back from there.

I have an ASP 91 generation 3 Sport Marine that I changed the timing on, put on a marine nitro quiet pipe, and 20% nitro with a head clearance of .010 and it runs as well as an old OS 91 Marine I used to have.

Just something to think about.
Regards Curt