I just re-powered my boat with three 300xs's earlier this year. My experiences thus far have been mixed.
When running, they are fantastic. My optimum cruise went from 1.4 @ 40ish to 1.6 @ 50ish. Noise levels are down, and speed is way up. I just ran to Bimini from Port Everglades, cruised at 61-62mph both ways @ about 4800rpm and I could listen to the stereo the whole way. Very nice. Holeshot is great and the visceral noises they make at 4000+ rpm give me a chubby every time I hear 'em running.
On the down side.....
1. I blew up my first power head (stbd) after only 10 hours, while at IDLE! It took Merc Racing nearly 6 weeks to supply my dealer with a replacement power head. (note: dealer had the order in the day after it blew up, and had the power head back on the day it arrived, so kudos again to Sundance Marine) Needless to say, I was pretty peeved that it took Merc Racing 6 weeks to supply a warranty replacement - thats 6% of my warranty period!
2. While over in Bimini this weekend, the power head gasket on the center engine (not the replacement!) partially failed and the engine started spitting water out the exhaust ports on the back. Not good. It would pull only 0.7psi at idle and thus would overheat at idle if not shut down or jumped on plane. Thankfully the gasket held together enough that it would still pull about 10psi at 4800rpm thus making the trip home more pleasant than it would have been on 2 engines. Sundance ordered the parts on Monday and - guess what - they are on back order!
3. The only engine that hasn't failed in some way (YET) is the port engine and, if you ask me, that motor is already slightly ill. Nail the throttles and the center and stbd rpm's rise in unison, port engine lags behind. Port engine requires more throttle (or trim) to maintain same RPM. Port engine doesn't want to reach max RPM unless trimmed to the moon. Throttle cable is good, throttle plate opening all the way, prop in perfect shape, etc, so its nothing obvious.
So there is my 0.02 on the 300xs motors. My 225 Opti's were 100% flawless and, last I heard, ktrain had already put another 150 hours on them without an issue. My 300xs's only have about 45 hours on them and I've already been out of the water several times, one major failure, one minor failure, and one possibly impending failure. Not good, IMHO. Who knows, maybe I've just been unlucky. I know folks that have 3x and 4x the hours on theirs without a single issue..... to be honest, I'd be less annoyed if Merc Racing readily stocked more parts!
Rick
Anybody Running Mercury 300xs ?? 
