Hell no, anytime you can dump an OMC for a MERC, your leaps and bounds ahead of the game. http://free.***boat.net/ubb/biggrin.gif
Havasu Barney reported that the 300 ProMax had more like 280hp whereas the 300X has more like 330hp. If this is the case (which I do not doubt) there is no comparison. The main difference I see is in high performance applications, the original 300 won't rev high enough. I think the limiter was set a 6,250 or so (not exactly sure). With the X, your limiter is either 6750 or 6800, so right there, you are better off.
In general, anytime something new comes out, it is best to let the manufacture work out the bugs before you buy. I believe you will be happy with the purchase.
What kind of hull is it going on?
PS: Snapper, sorry for the OMC bash, it was an open shot I had to take! I would expect nothing less if you were put in a reverse situation http://free.***boat.net/ubb/wink.gif