I have NEVER seen that dramatic off a drop on these tires after 3-4 runs...never! If anything they seem better as they wear in...Now I run on 2 tracks...a very high traction permanent facillity and a parking lot track that is "less than perfect" and I can get 20 runs easy. And mine don't show any problems until they actually wear out. I think we really scare newer racers off by telling them a 35-40 dollar set of tires/wheels will wear out in less than one day of racing...it simply isn't true. I don't know many newbies that can drop 80 bux worth on tires every race day.
Now granted, maybe at the highest level of racing where 1/10 of a second seperates the field...then yes, I'm sure it matters...but that's about it.
I have never had a problem spending whatever it may take to be at the top, and if it were truly 2-3 tenths off after 3 runs, I would definately keep new treads on all the time, but I have never encountered this...
As for inserts...I like the Mediums that come with it...a slightly firmer insert will help with the inner groove problem, as will running a 1/2 less neg camber, but I haven't seen that problem with a properly set up car. Shoot, I've seen the old cheap open cell foam inserts work awesome on a very cold track.