I don't see why anything would need to "tower over stealth", looking at SeaWorld's Mako (often rated as the best hyper coaster) which is shorter than Stealth. As previously stated Stealth has been marketed heavily on its speed as opposed to its height more recently and so I really wouldn't see this being a problem, and looking at the queues Stealth gets there's still very much a market for larger coasters in the U.K.
I guess it depends where in the park it was. Say we're talking the island behind The Swarm, then I would argue that having a hyper that only just
gets over the 200ft boundary would look a bit poor. Having a hyper that is not only shorter than Stealth, but behind it (thus having a greater apparent loss in height due to distance) would make it seem a bit lame - you have a coaster that would (presumably) be marketed for its height stood near enough to an even taller coaster which beats its height without really trying. If Thorpe get a hyper (and words cannot describe how much I'd want them to), it either needs to be far enough away from Stealth that the two aren't competing side-by-side or it will need to "tower over Stealth".
Mako is rated as the best hyper because it's usually one of the first hypers people ride due to its location. Coasters like Shambhala are considered much better by most people who've been on many hypers, mainly due to their extra height and speed. That said, most Thorpe Park visitors haven't been on a hyper either, so I guess any height would do in terms of public reception.
If the Thorpe hyper happens, I'd personally dream for something to beat out Thunder Dolphin as the tallest hyper in the world. Or maybe we could get a giga?