i've read that its recommended to run 275 30 19 on a 9.5" wide rim to keep a close running diameter to stock? or so?

but what happens if i run a 245 35 19 on the rears of my e60? (same tyres on the front swell)

the wheel specs are

19 x 8.5 +20
19 x 9.5 +20

I've already got a set of 4 245 35 19's and I'm currently running them on my e60 with m5 19's. (not sure if they're real so i need to see if the backs are actually 9.5 wide). - bought the car like this.

 

Use this link and see for yourself. I find it pretty handy.
Custom rims, wheel tire packages for your ride - RIMSnTIRES.com