Ok, My new wheels are 19x10 on the rear with a 43 offset.
here is a pic
Using that morfed tool, comparing my current wheel to your proposed 9.5 with a 40 offset means your outer edge retracts by 3mm (it sits in 3mm more than mine).
It's up to you whether you go down the spacer route. But it's been commented on loads, and the general consensus is that you should really just buy the right size wheel from the off. Spacers are not recommended.
Edit - as for the tyres, I was told I could have had a 265 profile rear on mine, but I went for the 275 as I thought it would give me an element of protection for the rim. But look at the pic, the rear tyre is still stretched, so god knows how stretched a 265 would be.