Interesting - as I have a 27.5 HP motor. By that, I've combined the block of a 25 HP with the carbs and ignition of the 30 HP. Jokingly, I call it a 27.5. In reality (and I've done this in excruciating detail), the part numbers between the two are identical except for a few key items: head, carbs, ignition module. On the head, I'm guessing it's the cams - and I have both heads - just haven't examined the lift or timing to see the difference. The carbs likely are identical except for the jets that are in them. Between the cams and the jets, Honda likely found 5 HP. If not, perhaps the ignition module is timed differently or has higher max RPM and could cover some HP. Not sure where they get it.
But, I've run both sets of carbs on my engine. I have two units - one with a bad block and the other with a bad lower unit. When I started the 25 HP engine, it ran poorly. Since I had carbs from the 30 HP already in hand, I disassembled and thoroughly cleaned them. I then reassembled and bolted them in place of the 25HP carbs. It started and ran great. So, it's clear to me that the parts are interchangeable.
I have an extra set of carbs that I'm nearly complete rebuilding. I was going to put them up on ebay and/or craigslist once complete. If you're interested, shoot me a PM.
freeportd
