A 275 should be more than fine on a 9" wheel. Tirerack lists a 9" wheel as the minimum size for every 275 I've looked at on there.
I'll give it an "OK", but no more than that. Minimum-width wheels in general says you're either working within some competition class rules that limit wheel width or that you care a lot more about appearance than you do about cornering.
I have 295/35 on 20x10 and 255/35 on 20x8.5
The 255's on a wheel wider than 8.5 will start to look stretched so I think 275 would be perfect for your 20x9 wheels.
8.5" is the "measuring width" wheel for 255/45. The T&RA and mfr tables give you another inch wider as being acceptable. The 2008 GT500 ran 255/45's on 18 x 9.5 wheels.
Where cornering response is concerned, a "stretched" appearance tends to be precisely the right look. Don't let old school and drag racing "looks" confuse this. The GT500 setup above works quite nicely on a NA 4.6L, though it's still understeerish over a 20 minute track session partly because 255 still isn't enough tire (and partly because -1.8° still isn't enough camber).
Sounds like I was on the right track. Out of curiosity, what effect would running the same size tires on staggered wheels have?
A wider wheel will stiffen the tire laterally and make for generally lower slip angles. When you are tire size limited for any reason, a wider rear wheel will help ensure a little understeer. This is as opposed to tire size stagger where you throw away front tire grip to get that understeer.
Even the car mfrs are starting to grasp this - Chevy's Camaro 1LE and the upcoming Z/28 both run staggered wheel widths but equal front and rear tire sizing. 1" difference in the 1LE, 1/2" in the Z/28 may speak to the relative levels of understeer intended.
Putting a narrow tire on the wider wheel (rear) would not yield the traction that a wider tire would give.
In other words, it kinda defeats the purpose of the staggered wheels.
For straight line acceleration traction that may be the case, but for cornering where you're generally trying to reduce OE understeer you're actually helping things along. Basically, it is possible to have too much rear grip, and crutching that with stiff rear springs and/or sta-bars is more of a Band-Aid than a true solution.
'08 GT coupe, 5M, wheels, tires, pads, fluid, a few suspension mods . . . still almost stock height
'19 WRX, 6M (hers)
'01 Maxima 20AE, 5M (spare, winter driver)