I'm curious to see what the life span of these tires is actually. I might be going with something different when these are done. Maybe I will jump to the MT's. :-D
You might be waiting a while. I have 17k on this set in 3yrs, and they had probably 60% tread rear and 75% on the fronts when I bought the car. The rears which are now on the front are ready to be replaced, but I still have decent tread on the rears. Im betting I could go another year safely on the rears, excluding racing of course, but just regular driving they are fine.
If I were you I'd just go ahead and buy some stickies for the rear and save the 555's for long trips.