Not anything but local club tournaments.
Way back in the 1980's when BASS was still on the 150 HP Max rule, I had a Ranger 373V Bassmaster Classic that was rated for a 150, but all the other 370 Series Hulls were rated for a 175...That sucked.
I would suggest just changing the decals on the motor.....Not that I am implying you would be cheating....

If this guy were to be involved in a high-speed accident that damaged someone's property, or worse...hurt someone....the prosecuting insurance company could rip him a new one.

I'm sure a lot of guys change decals on their cowlings. It's really not a big deal unless, of course, you win...there's that pesky old polygraph question: Did you knowingly break any rules or laws?
Ouch.

So, how does the insurance handle 20' boats with "unlimited" horsepower ratings if some one puts a 300+hp outboard on the back? It is a legal set up, per the Coast Guard.

If it's legal as per the Coast Guard, I would imagine insurance companies consider it just fine.

I had an old ProCraft 1650V that was rated for 135HP and had a 'rude 150HP on the transom. I got insurance with no problems. Only fished club tournaments and local tournaments.
Couldn't fish any tournaments that had a horsepower clause.

Depends on the Ins Co Henry. Mine didn't care what I had on back of it or what the boat was rated for. All they wanted to know was if the boat would exceed 45 mph...

Progressive on the other hand had an 8:1 formula for boat length vs. HP. In other words if the boat was 20' long they would not insure it if it had more than 160 HP. Pretty silly but thats what they told me. That's when I switched INs Co's.

I have a 150 on mine and its rated for 175 I'll swap you if your worried about it.

If they did...lots of proXS owners would be over the rating. 225 puts out 249hp, 250 puts out almost 300hp.

The only time I have been questioned by my insurance company was when I bought my first Bullet. My agent with State Farm asked if the boat was capable of 90 mph. I cancelled and went with another company and have not had any issues since, even with my current boat. I don't think the rating makes much difference since horsepower is unlimited for boats over 20' in the eyes of the Coast Guard, but an insurance company can make any rule they want, so can a tournament director.

I thought the actual hp had to be within 10 % of the rating. Where did you find those #s.

on the tournaments, I have asked 3 directors about this very question because I have a johnson 175 fast strike that has been worked over by monty racing in fl. and dynos 215 at the prop. all 3 told me that if it is a 175 then what was done internally makes no difference. these were all 3 from the bigger trails here. but my champion is rated for a 175 so I am legal per the coast guard.

swap someone out for what your boat is rated at and go fish.

Simmons Custom Boats in Kemah has posted dyno runs in the past of a 250XS and a 300XS. The 250XS dynoed at 274hp and the 300XS at 305hp. I did not see the test, but they have published it in their test. The 225 Sport XS was dynoed at 247hp.

It can't be over the horsepower rating to fish tournaments