They say only about 1% - if that's correct then it's a lot of development and cost for a very small % of riders. I suspect though it's currently higher than that as too many riders have too high a seed/ZRS and they want to reduce it to a value that reflects where they really are (I'm definitely in that category).
Thing is though, it's still easy to tank ZRS - just let the pack go in the first minute, then ride hard as a solo rider for the rest of the ride - result is a decent effort in power/HRM but a poor finish position as the pack will go much faster than a solo rider. This is undetectable as getting dropped from a pack is par for the course in racing.
I still think Zwift are trying to solve the wrong problems that they have created with ZRS.
Another undetectable way to lower your ZRS score is to stick with the pack until 5km or so until the end. For most races, certainly flatter ones, this gives a decent 30-35 minutes of effort. Then for a few seconds just lose the back wheel and let the pack go. Raise the power again to tempo power and the pack will continue to pull away and your power numbers and HR will still look like you've raced hard because for all but 20 seconds you did race hard.
Result, there are loads of riders ahead of you and your ZRS reduces. And you had a great workout.
I can easily see some racing hard on a Tuesday in leagues, winning a race, then during the week getting good workouts but reducing their ZRS to allow them to win a race the following Tuesday.
Zwift seem to be looking for riders who ride Z2 and cruise around a race as an example of tanking their ZRS, but the reality will be that they have actually made it easier with ZRS to manage the score if someone so wishes.