yeah improved tracking will scale forever just at a reduced rate. So you could theoretically sit there and farm it for a couple of days and get another completion done probably.
the normal soft limit is +100%, the soft limit in the challenge is thugs +1000% or 10x. or at least I'm pretty sure it is. You can tell if you're at the soft limit or not by going to the stats and info screen and then looking at statistics and wave clears. It soft caps at 1000 wave clears, so if you have less than that it's a a ways to go yet to the soft cap