To lower the compression, you'd have to change the internals out to low-comp pistons, etc., so you'll see no gains with going with a B18C5 since you won't have any use for its forged, high-comp internals.

This is why it's cheaper to just go with a B18C1 for forced induction. Do a search and you'll find a few threads about this along with some more indepth reasoning.