Have you actually put 19's on your Zed to back that up?
I run 19's with MPS2's and there was no difference what so ever after fitting them. A good tyre and a full alignment will make no difference to drive ability.
And "Go factor"... it make no difference at all. Put a Zed with 18s next to me and he wont get anywhere faster.
Maybe you should be more concerned about the weight of the rims themselves, rather than the diameter.
And yes 19 x 10 ET-7 will be fine with a rolled arch, mine on the rear are 19 x 10 -6 IIRC.
My 18" lmgt4 make the car alot more responsive than my 19" grids, but grids fill the arches better
What is the weight of the Grids, what tyres were you using? So much more to factor in that just a simple wheel change.
Again, my 19's made no difference, but we are talking about a completely different wheel, setup, etc