Here is a thread titled "Fuel consumption with the big block engines" that I think you'll find of interest.
Lots of good data here.
Generally, any motor big enough to push a boat at a given speed, is going to have very similar consumption to it's equivalent.
The new big blocks with fuel injection are impressive, but it's going to be quite a while for that efficiency to pay back the difference.
As with diesel, same thing, tough to justify on a Commander this size unless you have the cash and just want to do it, which is a valid reason to do it, but it's not financially justifiable to most situations. Diesels are best used constantly, kept warm and on the run, lots of maintenance, and it's somewhat of a myth about the benefits in a boat this size.
In any case, here's a good thread with some info you'll find of interest.
http://www.network54.com/Forum/424840/message/1150656733
regards,
Paul