I have an offset driveway, and the other day I parked the car on the slope, one side of the car dropped down so that the tire was tucked into the rear wheel well. This gave me a really good idea as to how far out I could come with the stock 275 tires.
I only had about 10-12mm clearance (I have 15mm spacers) before the tire would start to rub on a really hard impact or incline. So I think max you could go is 25mm out and be safe.
I'm still considering running a 305 Nitto Invo on my stock 10' rim, or at least a 285.
So I'm going to stick with a 20mm spacer for now.