NB,
does the amount of warning affect the accuracy as well as how much time you can gain/lose? For example, if instructed to lose say one minute while already on descent (I know we shouldn't) is that harder to get spot on than say 2 minutes but while still in the cruise?
We either can or we can't. It's all a case of what the FMS says: if it says the FF time, then the aircraft should get there on the time provided the crew fly at the speed the FMS wants.
So the short answer is: no. If we get told to speed up/get our FF time brought forward, it's simply a case of seeing if we can achieve it. We can or we can't, no "accuracy", per se, involved.
Something else to consider (may have been brought up earlier in the thread): our FMS only indicates to whole minutes (at least until the last waypoint before the FF). So theoretically, an aircraft with a FF time of 12 could get there at the back end of 12, and a follower could get there at the front end of 14. That's quite a difference in separation compared to the other way round. Some of us adjust the descent speed to arrive just after the minute changeover.