1990 Nova single 7.4 w/ carb. The boat came with a 23 pitch Mirage (likely from when the boat was new) which looks old but is in ok shape...couple nicks and one of the blades has a very minor ding. WOT rpm's are @ 4400 give or take. I am considering upgrading to a Mirage plus, would I just get the same pitch? I don't know all the diffs btwn the two but I've read the plus has more cupping.

You would probably need to drop to a 21P.

Yup, most likely 21P

Really...is that due to the cupping or is there more to it than that?

A Mirage Plus will spin about 200rpms less than a regualr Mirage.
It sounds like the current prop could use some minor fixing. It could also be out balance.
Send it to BBlades. He can tweak it and basically make it into a Mirage Plus and lab it to keep the rpms up.

That is an even better idea

Yes the M+ has more cup which drops the rpm about 150-200. If the Mirage doesn't blow out and/or ventilate while getting on plane and in turns you may want to stick with the Mirage and get it "touched-up" as Griff said.