Why not just try it out?
Revving the spin dash definitely makes a difference.
Here's a non-revved spin dash:
And here's a revved spin dash:
Here's the drop dash:
The drop dash appears to be slightly less powerful than a non-revved spin dash, but note that the jump was on a slight hill that caused it to start off with backwards momentum, so they might actually be even.
Here's a super peel out without waiting until it's fully charged:
And here's a fully charged super peel out:
Normally you'd continue holding right to run at full speed after a super peel out, but without holding right it looks about as powerful as a non-revved spin dash.
Also note that holding right actually slows you down when you're in ball mode!
Non-revved spin dash, holding right:
Revved spin dash, holding right:
Drop dash, holding right:
As for how many revs gives you diminishing returns, I think the key is to listen to the sound it makes when you add a rev. Each one causes your "revving noise" to become higher pitched, but only up to a certain point. I think that number is about 7 revs.
Anecdotally, I've noticed that speed-runners jam on the buttons and rev it a bunch of times. And it might be worth noting that in Mirage Saloon zone, the plane will keep up with a non-revved spin dash, but if you rev the spin dash then you'll fall off the plane.
If somebody knows of a better straightaway for testing, I'd be happy to try it out.