Ilmordude, I beleive you are on the right track. Miklos offered up a 525 to dyno and log results, you should probably jump on this and log data, even if just for information purposes. If P1 power to weight rules will apply for SVL next year this is something you would want approved ASAP. The SVL class right now appears to be drawing quite a bit of attention and more boats, including new boats will be entering the class by year end. With that said, the only option right now is the Merc 525. New boats coming in this year will need to purchase the Mercs with the current rules. Chances of a racer buying a 525 to run this year and pulling it to buy an Ilmor for start of next season will be pretty slim. I do not think the racers budget for this class will allow it. I, for one will be entering a new boat this year and most definitely would run the Ilmor over the Merc. I would prefer to have the extra weight savings of the motor built into the construction of the boat.
One thing to keep in mind is that the Merc 525 is an extremely stout race motor with a great reputation for withstanding many trouble free hours of WOT hammering race after race.
Building an engine in the 570HP range may do well in the high performance/poker run arena as well. Many boats out there are running the Bravo drive which in most cases limits the HP to 550/600. Most of these owners turn to the custom engine builders to build GM based engines in this HP range.
As far as the SVL, you may want to procure a copy of the current rules and speak with the leaders of SBI, OSS/P1 and OPA. From what I understand, the boats in this class regardless of the sanctioning body registered with should be able to compete with each other under any of the above banners and fit into each set of rules.