See the link to the Weight Training thread.
Mostly experience with juniors that lift vs juniors that don't. The lifters have occupied the top step in the ~30mm VOS TTs against another 70 or so kids. There are also very talented and winning riders that don't lift, but they seem to have put double the time into training. The lifting is for time efficiency. I'm not going to argue doing on-the-bike sprints won't work, just weights work very well.
I have many, while unscientific, data points in the :30 range, I really have just my kid for the hour. If you believe your FTP can be calculated off of :20 min power - sure. I'm not a big fan of calculated numbers, but I would expect some benefit. There is a video somewhere of Sagan doing leg extensions. So he thought it had some benefit to longer rides, likely for the finish, not FTP (this thread), same as other World Tour road sprinters known to lift.
If riding alone, maybe hour FTP increases (which is the topic) may be small, but if in a group, it is the extra power 15 sec at the end that moves you to the front that matters most.

As to adding mass. The lifting juniors over 4 years did not significantly increase mass and increased W/Kg. The kids grew from 14 to 18, but maintained about the same proportions and looked like their peers who didn't lift. The lifters also had less training/over use fatigue that was/is common to juniors putting in the big miles. Other elite kids did not lift, but they did/do spend much more time on the bike than the lifters I know.
For "fun" find the gym kid/s here. I know some of the lifters here and some aren't and have no idea on others (my son moved on, and is not pictured). It is very hard to tell by looking. LUX Cycling Team