shit. Is that what less than vertical means.
Surely achieving a mild pump would be much better on a 10deg overhanging wall about 3 grades below your o/s limit?
Get a bit pumped but not so much that eventually you'll be too pumped to continue ie pump should never rise above a sustainable level. Toproping slabs sounds unlikely to be demanding enough no matter what your level. Really you should get an answer off Serpico if you want an exact explanation.
Conversely, I've recently taken to doing slightly shorter stints on the wall at slightly higher intensity. I've been trying to stay within the range where although I'm a bit pumped, my arms are dealing with the lactic acid at about the same rate as they're producing it. I have no research which says this is better, but it's ever so slightly less boring. And Abarro81 does it.