I just looked back through my pics of H in the Monarch, Parkway, Bodyguard, B510, AirBooster and Start (hope I'm not forgetting any), and really none seemed to position the lapbelt where I would consider "too high", but like Kecia said, that can really depend on vehicle belt geometry. Vehicle belts that are anchored forward of the bight will naturally fit lower on the lap than vehicle belts anchored in the seat bight. I do think there has been a time or two I noticed the lapbelt riding too high on H in both the Parkway and B510, though I can't remember which specific vehicle.
Like Tiffany, I always make sure H has pulled her shoulderbelt enough so that the lapbelt is nice and tight, and my Windstar has locking latchplates so I don't worry too terribly much that it will loosen. However, the Bodyguard is wonderful in situations without locking latchplates. That's the booster I sent with my mom over Thanksgiving when I was out of the state. Her vehicle has locking shoulderbelts, but I felt much better with the BG's lapbelt lockoffs, too. I figured it was the best of both worlds, locked lapbelt and shoulderbelt.
The lapbelt lockoffs position the belt DEFINITELY across the upper thighs versus the lower belly. I really, really like the BG, the only problem we have with it is quick pick-up/drop-off, like at school, because H can't consistently lock the lockoffs herself.
I liked the positioning of the belts with the Graco AirBooster and Monarch quite a bit, and the AirBooster offers more leg support for long-legged/taller kids, so I'm leaning toward that one in the future, but I also love the Recaro Vivo because of the "cut-outs" for the lapbelt. Seems like that would hold it in place very well and prevent submarining. But the Vivo isn't very deep.
On the subject of using the Recaro Sport reclined in booster mode, is that allowed at higher weights? I know the recline mode of our Recaro Start must be discontinued at a certain weight (don't remember offhand, I just remember H is already over that weight so she must use it upright).