Hi Cbass,
Welcome to the forum. Just a heads up that both Peng & I recently purchased a Denon AVR-X4400H, so we are up to speed on this. (For my setup see
Denon AVR-X4400H - Initial Setup ) The 4400 has 9 channels of amplification, but it can process 11. You can connect an external amplifier for any pair of speakers you'd want to drive. Even though I installed 4 in-ceiling speakers, and moved the rest from a 6.1 setup to a 7.2 resulting in a potential 11.2, or 7.2.4 in Dolby Atmos parlance.
My logic is I wasn't sold on the impact of immersive sound using 13 speakers for my room (which is of similar size to yours),and difference in price for a Denon 4400 and the Denon AVR-X6400H; was the 4400 was about half the price of the 6400. So I opted for the 4400, and plan to buy 3 HSU CCB-8 speakers to replace my aged fronts with the difference. There is an App you can get for the Denons which allow you to save different setup configurations, so you can easily go from say a 5.2.4 to a 7.2.2, etc.
Another concern is the physical difference between the rear surrounds, and the rear in-ceiling speakers in my room (which is similar in size to yours) meet Dolby standards (at least as far as I know),but they certainly seem close to each other. I'm really not certain how impactful a 7.2.4 will be versus a 5.2.4, or a 7.2.2 with only the front in-ceiling speakers. I think you too should consider this.
However, if you've got $2500 and want the simplicity of an all in one AVR now (which will be more convenient) I'd suggest the Denon AVR-X6400H. If you are in the USA, and not adverse to B Stock (I'm not) try this $1600 USD deal at
Accessories4Less - 6400H Deal If you are in Canada a similar $1900 CAD deal is at Gibbys
Gibby's 6400 Receiver Deal
I hope this is helpful.