Thanks for the perspective guys. I’m leaning towards the 9 at this point. I’m wanting to cut my 7mm mag down or re-barrel it at 18-20 inches. That would make the overall barrel length w/suppressor only 3 inches longer than my current 26 inch barrel and a lot more pleasant to shoot. If I could get away with an 18 inch barrel and the ultra 7 then I’d be an inch shorter than current length. Either way I’d probably have to always shoot it suppressed at that length.
Interesting project, you're probably going to wind up with a package very similar to my 18.5" .30-06. With my stout accuracy load, it puts 180gr Accubonds out at 2,690 fps. Really a sweet shooting rifle with my can on, but uncomfortably blasty without. I don't shoot it without my can and i'd bet you won't really either, that's ok though, the whole point of chopping it to me was to suppress it.
Here are some pics that may help with visualization of your project.
That's a Liberty Sovereign, so a hair less than 7 inches added on when it's mounted.
As for the suppression level, keep in mind that the muzzle numbers aren't really the most important for a recreational shooter/hunter, the at ear numbers probably are.
I had an opportunity to do an impromptu metering of my setup on a B&K 2209 at the range one day (thanks to a certain generous local suppressor manufacturer). My statistically shaky sample of a few shots each showed ~142 dB a meter away from the muzzle, but ~135 dB at my ear.
A bit over the "hearing safe" threshold at the muzzle, but good enough for me at the ear. I'd guess with the higher volume of higher pressure (at the muzzle) gas in your 7mm Mag and the larger differential between suppressor bore and bullet diameter, your setup would be louder. The at the ear numbers might still be good enough with a 7 in can to opt for the shorter length... or maybe not, just something to consider.
You've probably already seen this, but if not, it should give you an idea of the velocities to expect at different barrel lengths in a 7mm RM:
https://rifleshooter.com/2015/04/7mm-remington-magnum-7-rem-mag-barrel-length-versus-velocity/