I think we're all generally stumped on this one as you've described it. Each outlet has its own volume control, and there's one thermostatic valve feeding both. The shower head is fine, and the cold
water is fine at the tub. But the hot
water at the tub is weak?!?
My only guess at this point is that the shower head has flow restrictors that make the problem virtually unnoticeable there. But the tub spout is full flow, so when you advance all the way to max hot, the drop off is obvious. If that's the case, you'd most likely have to replace the cartridge - although - Grohe valves tend to have lots of screens inside; you may pull out the cartridge and find "stuff" clogging the screens on the hot side?