I'm running a 250ProXS on the Black Cat. Its a good motor, but it came from the factory with a couple defects.
1. It got a new computer before I ever put it on the water because it would not drive the SmartCraft gages. The installer said it worked fine, and it must be my old gages because it worked fine when he installed the motor. I wasted money on new gages before he gave me a new computer. (I wasn't going to take it out and break it in without a working tachometer.)
2. At 13 hours I got a brand new powerhead because a piston sleeve spun in the block. The first diagnostic indicator to me was that it "hunted". I had fired up the motor on the hose before taking it out to the river and found the gages didn't work. When I went to the dealer to get the new computer installed I told him the motor seemed to hunt excessively. I have owned a few high performance injected motors and I am familiar with the fact that some of them seem to hunt a little at idle, but this seemed excessive to me. The dealer told me it was normal and wouldn't even fire it up. (Not even after he put in the new computer which was done in the driveway.) I took it out and ran it, and it seemed to "miss" periodically. It was a sudden minor jolt almost like tagging a sandbar with the skeg, and it was not consistent. When it kept doing it at any speed including WOT after the ten hours of beak-in. I called a different shop, and there it sat for months while they diagnosed it. I got a brand new powerhead.... eventually.
Before that the Cat had a 225, and it stuck a piston at about 30 hours for which I got a brand new powerhead from Mercury. Both were covered under warranty, and both took months to get done. A buddy of mine is running the 225 on his boat now, and the 250 fires instantly and runs like a scalded cat (pun intended). Do I love it? No. When it basically blows due to a defect out of the box its hard to ever trust it again, but it will light up and go. In good conditions it will push the Black Cat as fast as I can drive it.
The morale of the story I guess is if it doesn't seem right make the mechanic make it right. If the mechanic treats you like you are stupid get a different mechanic.
I think Mercury makes decent motors, but I will never be the zealous psycho fanatic that says things like, "once you go black...," except perhaps in jest.
One thing I would suggest is to pop over to Bass Boat Central and look for people who have that motor on the same model boat you want to put it on and see how they set it up and at what elevation and humidity they typically run at. And don't expect to get the same numbers as anybody who's claim seems pretty darn good. I won't say bass boat owners are liars, but I expect a lot of those numbers could only be achieved with a perfect prop, perfect setup for the conditions, down current, with a tail wind, no load, 12 inches of chop, and just enough gas in the tank to get back to the marina.
Enjoy your new motor and keep your eyes open.