A 15:1 compression ratio sounds a bit high for boost. In a watercooled four cylinder auto application I've seen CRs as high as 11:1 boosted to 20+ psi with methanol. I want to say I've even heard of a boosted 14:1 Honda motor but I guess things like your cam profile will ultimately decide your dynamic compression ratio. I don't think there is a certain right answer here because your tune will also greatly affect your detonation threshold.
I really shouldn't even be trying to answer this. Where's LeFevers at?