As you say, the ratio isn't ideal, but to be honest, I can't see it making a huge difference,
I wouldn't really expect it to be any slower top speed, assuming you get the right prop, but you may suffer a little more torque-reaction in the boat with that ratio.
You say it has a small block, but you haven't stated what power you think it is.
Is the bottom clean? (antifouled, or nice & shiney)
Which R25 is it, SC or mediteranee? I actually think that if it's a 260, or 280hp small block, and you're seeing mid-late 50's, you're not doing too bad.
"I Agree with everything you say really!" - John Cooke to Jon Fuller - 26-01-2013