I'll run an overnight test. I'm assuming you mean normal start positions, so : 8-moves book, 15s + .15, 2.4 with nets vs 2.4 no nets. I'll post results tomorrow.
I think almost all the elo gain is in the endgame nets but haven't verified that. Probably something like 8-moves book self-play is one of the best cases for showing late endgame improvements. Even if it is near +20, that still amounts to less than half an elo gained per day overall, and probably less than 1 elo per day even in the general endgames.epd starting positions. If I hadn't broken it down into the individual small endgame suites in the beginning I wouldn't have known it was working, except to go from very bad to passable after some training. So I decided to take a break even though it was working since it would take quite some time (in my initial excitement I had ideas of reaching the limits of training the endgame nets and finding optimal structures and processes, then working my way back toward the midgame then opening, but the space grows much bigger quickly.)