A Pro Trak can tell you how your existing cells are doing in a pack, but to be able to re-match you need such a large amount of cells on hand to be sure you have another that will fit right in with the rest of the pack you want to insert it into.
I have a 4 cell pack which I was testing about a month or so ago and found that one of the cells was dropping off earlier than the others, maybe about 15 seconds earlier or so. However, because I mainly run 12th stock, I never get into that last 15 seconds so it's not an issue for me, that pack untill that point is really good. What I'm saying is you need to consider how you are using them.
I usually find that the voltage improves a little after 10 runs or so, but the run time goes down, my 428's are now 412's. Even so, I'm still not getting within 30 seconds of this. When I come off the track my voltage is still around 1.15 per cell.