Interesting - I managed to dig up the website below yesterday, which suggests that the US cars had wider wheels with the same tyres - that would fit my requirements slightly better since I'd only need to change 2 wheels. Also I've just checked the manual, and it's definitely 255/40 on the rears, although I agree that makes it look a bit more jacked-up than I'd like.
http://www.wheel-size.com/size/lexus/is/2008/#trim-25-eudm
The point is I'm trying to preserve the warranty, since the car is still under ten years old. I've had £1150 worth of parts replaced under the warranty so far with no quibbles, so it's worth the £800 I paid for it. However I need to get back to OEM tyre sizes to ensure that any suspension problems that crop up can't be attributed to my wheels. Also the scraping is really annoying.
OK then, offsets. I can get the same wheel in 8.5j fitment for the front, in offsets ET20, 30, 35 and 44. The US cars apparently came with 8.5j ET40 on the front, and I want the wheels to sit slightly further out than standard, so they're almost flush with the arch rather than recessed. Should I go with ET35 or ET30?