For me (I guess that's the important bit!) and my experiences with HFT. Thumb up works very well on the straight and level (I.e practicing, zero ranges, some courses). But on angled shots can cause a bit of vertical wandering. The best explanation I had was from the main man Jim who described the thumb as basically acting like a spring which will absorb some of the recoil, then push it back into the stock. Possibly the different angles means that 'thumb spring' is acting in a different way?
The idea behind 'thumb forward' is that the thumb is to the side of the action, so the stock will slide past, essentially removing the 'thumb spring' from the equation.
The main issue is that every individual person, stock fit, rifle tune/characteristics, shooting discipline, conditions etc will probably require different hand placements to really get the best out of them. I've known people with standard 30mm thumping pistons hit everything they looked at, and other people with fancy 22mm or less tunes and target stocks not be able to hit the side of a barn.
For me, shooting a springer consistently well boils down to 4 main things. Stock fit, hand placement, trigger control and fitness/conditioning of the shooter. Find someone with all that and they'll win with anything regardless of rifle or tune.
I've by no means mastered it yet, but maybe next year