you should've got suggestions for a new prop/pitch/diameter from the engine seller. Also, the rpms are important for full speed. With my 2gm20, a 16D 11P would only give me 2600 rpm at full throttle and I'd achieve 6kts, but I could not beat against any strong waves without dropping to well below 3kts speed. I went to a 15D 11P prop and now gained 800 rpms, and now can achieve over 5kts against strong waves no problem. So all that said, how many rpms are you getting now with the new motor. If you're reaching above 3000rpm and no speed, my guess is your under propped, and of course if the opposite and can't get more than 2500rpm, then over propped. Either way you need to work with both the diameter and pitch of the prop to get the best results. I haven't looked into it yet, but like you, if I installed a new motor, I'd go 25/30hp as well, but just off the top of my head, I presumed I'd have to go with a bigger diameter than 15" and find the appropriate pitch to match, and expect the engine seller to provide the info for his prop in my boat. Good Luck!!!
