Claim of 25% HP bump true?

[47topless]

Just purchased a nice '31 Victoria. My first "running" Model A. Long in FH Ford V8's.
Going through a local Dealer's parts catalogue, I noticed a claim of gaining 25% HP with their milled, high compression head. While I do not question the integrity of this company, or this offering, it seems too damned easy!
Can the increased compression, achieved in such a simple way, really bump the HP to that incredible degree?
Phil