I have a 38 Fountain that I switched from Mercury 525's over to 625 Ilmors. I like the Ilmor engines. They are of sound design and reliable once in, but if I had it to over again I would go out and buy a boat with factory installed Ilmors rather than do a Merc to Ilmor conversion.
I had a well know racing rigger do my Ilmor conversion and my gwad it was a nightmare for them. They estimated that it would take 3-4 weeks and it wound up taking nearly 4 months to install the Ilmors and both of us wound up taking a bath on the rigging costs. The Ilmors didn't fit the stringers, they had to reglass the transom and raise the drives to get them to fit, my once perfectly dailed in X dimension was way off once the Ilmors were installed (taking 600 lbs out of the back of your boat may not always be a good thing) so the boat didnt' perform as hoped and is still in need of tweaking.
Would I buy a new or used boat with Imor power - Yes, in a heartbeat. But let the factory do it where they can set it up and balance it properly from the get go. Would I convert a Mercury powered boat to Ilmor power? GWAD NO!!!