Hitting 40+ in the lower cats is almost easier as you are racing most of the time under 25 mph and the races are shorter. I often hit 42mph+ in sprints as a cat 4, and I had a teammate that did the same so we would usually trade off and leave the other off at around 40 with 150 meters to go ... the leadout could coast in and get 2nd/3rd after that. Then I upgraded and found that I was winning sprints at under 40 mph in the cat 1/2 races and never really figured it out until years later with a power meter. Something about those near 30 mph average races kill max speed for me and holding wheels at a 35mph leadout with 1k togo also make it very hard to have anything left for the actual sprint. These are very flat races of course and the average is under 30, but the last few laps can all be over 30mph.
That said, I would be happy if leadouts in some of my races would just stay over 30mph but the numbers above do seem on the higher side. I just had a race where we were at 30 from km2 to km1. Then dropped to 25 mph from 1K to 500meters, sucked big time. I would say leadouts over 30mph in the lower cats is enough to keep people back until 1k for sure, after that it just depends on who is at the front and who wants to pass who.
In most races at the master's level I am happy with a build to 35mph at 500meters and then if it is anything under that someone is very likely to jump early, I would say somewhere around 32mph is the min you want in the 1k to 500m range. If you can have a sprinter jump at 400 or 500 meters and bring up the speed over 35 I am very happy if I am near the front. Starting my spring at 35 at 200meters to go is perfect. If the pace is closer to 40 at the end I would have to wait until 150 meters to make my final jump.