I'm running a 27 Advantage victory with a 454 mag and a procharger approx 575 hp .I replaced my engine coupler 3 years ago with a sierra brand . Well I'm being towed back to the shore as I'm writing this post lol. My alignment is good and I greased coupler yearly .
What coupler do I need ???
Thx

Obviously it wasn't aligned as well as you thought. I am running a stock Mercruiser coupler behind 800+ HP
What year is your boat. It is possible your rear engine mounts have sagged. The tool will show the coupler is aligned but it isn't.

get the merc heavy duty one and make sure the transom and stringers are truly solid.

Boats a 2001

people think it needs realigned cuz the drive came off or engine came out. nope, it needs realigned cuz it is made of fiberglass and wood. they shift with time. it doesn't have to be rotted or wet for the mount pad to compress 1/4 inch or so.

Just pulled the outdrive off the splines on the coupler are stripped

2 finger pull out with alignment tool with it greased so you can look at the patterns the splines left on the grease. the patterns have to be equal all the way around and full pattern marks plus the 2 finger pull out of the alignment tool.

If this is not achieved, your alignment was off to begin with.

Also the wrong grease use can cause coupler failure. The 2 main reasons for coupler failure right here.

Some of Sierra and Merc couplers are made buy the same manu's exactly the same Not all but some so most likely yours is the same Merc to Sierra / Sierra to Merc.

Have you had to adjust the front mounts in the past?
If you did what direction did you turn the adjuster nut - up or down?
If you had to turn it down for the tool to align than you might have a rear mount that is sagged.

Does it have aluminum or steel splines? I believe that the alum ones are just rated for 400hp

What physically happens when a coupler lets loose? Like freewheeling in a car (ie neutral)?