Yes, that is essentially the idea: By spending CP, you could give your (lower tier) ship additional weapon, console or Boff slots, up to the maximum of your ship type's best tier 5 ships.

Now, this should of course be more expensive than just buying a new ship of a higher tier with C-points, but it would give people the chance to have those "tier 5 Constitution" or "tier 5 K'tinga" and what else people have been talking about. It would of course still not increase the hull or shield hp of the vessel (and I would keep it that way), but be a good way for the sentimental among us to keep their Miranda up until tier 5.

What do you all think about that idea?

An interesting idea, and I would be even more interested to know how CBS would feel about Vice Admirals flying Miranda class.

Well, they should feel about the same about there being thousands of admirals in starfleet.
Their worries of canon and the reputation of IP change drastically on whether it suits them or not.

Well, in canon, the Reliant was definitely on-par with the Enterprise.

I doubt, though, that many would want their Miranda upgraded.... their Constitution or Akira, however... I can see on the forums that people would do that. And pay C-Points for the ability to do so.

No, it wasnt. The Enterprise would Spank the Reliant, the only reason it looks that way in the film, is Khan crippled the Enterprise right off the bat, with the enterprises shields down.

"They knew, exactly where to hit us" - Spock

I don't want to go too deep into that tangent, but see the Memory Alpha article on the Reliant.

I have seen so many different requests here for Tier 5 versions of smaller ships (mostly Constitution, Akira and K'tinga, but there were also voices to update the Sovereigns for "Tier 5+") that I cannot count them.