Nothing extra that I am aware of. It is not a true race engine so still has a warrenty etc. It is pretty much the same as the Optimax. The main reason they sell it as a "consumer race engine" is because it doesn't meet all the latest environmental laws so they can only sell it to race or commercial users!!!
Well it's bound to help - engines are about the same weight but the Mercury has 50 or 60hp more. Having said that it is still only 20% more than you have now which isn't a massive amount. My engine is a 225 so I stand to gain more - to be honest if you are happy with the engine you have I would wait until you HAVE to change it - not before!!!
Do you think it would greatly improve my performce over the 250 DI?
There is an other "important" performance factor besides more HP, namely the gearcase. The sportmaster gearcase is designd for better speed performances. Only this can already increase tops speet with 2 or more kn. But I'm sure the racing people over here can give you more information on this.
You can treat the XS range the same as the Optimax's - they don't have as many of the sensors built in, but the smartcraft system still works - one thing though they don't come with controls or anything - check with the people you're buying from what you'll get!
They are also in short supply so you might have to wait!