I agree totally Jacques they're very good engines, the thing that puts me off (other than the price tag!) is the fact that when they do go wrong they can't be repaired with some string and some araldite.
It would also mean completely re-setting up the boat to cope with the extra weight and ponies that an XS gives you.......Also not forgetting that there's nothing different between my powerhead and any other (std) Merc carb motor- 'Mariner' is just what it says on the cowling..... a 200 HP 2.5l carb powerhead is what's underneath. The only real difference is the leg/bracket which all appear to be made of cheese.
Overall though, we built the boat to be low-budget and fairly competitive. Bolting £8000 of motor on the back (though awesome it would be!) would reduce the enjoyment i get out of the boat- i'd be constantly worried about losing/ drowning the thing!
There's a P25 close-by that needs another 200XS