I currently the very happy owner of a 24’ Superboat Millennium CC w/ 300xs on a bracket. Love the boat.
We do routinely get fairly rough conditions. 17 knot winds opposing 2.5k knot tidal currents. Also when we head over towards Hilton Head or Bay Point island there’s exposure to ocean swell. I don’t mind a bit of “air time”, but my wife prefers the plane when it comes to flying.
So my question is would the 30’ version of my boat have a significantly better ride? Does ride quality improve in a linear fashion with length for a similar hull design?

You are not going to be "comfortable" in 4-6 foot swells in a 30' boat. But, the additional 6' feet is going to be a nicer ride. You use the length of the boat to allow it to slice the water open in a landing. If you have the means to do so, I would recommend the upgrade. Just make sure that the deadrise on the longer version of your boat is the same or even more acute.

Night and day difference that 6' makes. More so in the slop of the bay areas and general ride. I went from a 22' Donzi to a 29' Kryptonite and it was incredible how much better it rode. Now I like the ride of my 38 and could never go back.
BTW - 4-6' swells in a 38-40' are not great.

I owned a both a 24 and a 30 Superboat. The 30 is a much bigger boat, Y2K's are a little over 31' loa, the 24 is 23'7". If you follow racing, you will see that Y2K's win a lot of races, particularly the rough water races.

I owned a 24-foot sunsation, similar 24 x 7 hull.. and jumped up to a 29-ft warlock, and now 30 ft activator... You're going to notice a huge difference in the overall ride no matter what conditions you are in. Like someone above me said if you can afford to make the jump I would definitely recommend it!

I find my wife has less of an issue with rough water when she's driving. I'll be doing 70 and it's too fast, so I let her drive and we're doing 80. Maybe the solution is to switch drivers.

Just out of curiosity do you know where a 30 ft SB is located? I haven't seen any for sale.

Theres a Y2K in the classifieds. They are bad a$$ boats.

Haven’t started searching yet. I’d be wanting the CC version.

I'm sure you can get a new one built. Good luck finding a used one, not many out there.