Did you manage to get a clip of the exhaust Mark?
With regards to Lexus warranty, only the modified parts i.e. exhaust will no longer be covered, everything else still will be covered - other than any issues they can directly attribute to having a non standard exhaust I would guess. I'm sure Stu checked this and posted the info on here although it was when he still owned his F so a good while ago.
I'm in the middle of making enquiries regarding an exhaust - just need to find somewhere reputable and reasonably local. I'd have an H&S system in a heartbeat but the logistics are just too difficult at the moment with their distance from me even though Ian has said he can probably fit it in a day.