I'll make this short, pending Jamie Goode's entry into the thread.
There's good short-term data that riesling holds better and more consistently under screwcap, but that the wines do age as expected. There's some long-term data that indicates, more strongly, the very same thing. There's a little bit of very long-term data that further demonstrates the same result. Mostly, the idea is that rieslings under screwcap age like they do under the best corks, with very high consistency. (I don't believe there's any evidence that this effect is restricted to riesling, either.)
Most of this data is from Australia, and I'm sure Graeme or someone else could point you towards the data, though a little Googling will probably reveal it as well.
However...
There's also some evidence, though I think it's all weight-of-anecdote at this point, that screwcaps increase the likelihood of reduction in certain wines, which effect would be increased over the long term, and thus prove damaging to the wine no matter the qualities of the screwcap. I don't know if riesling is or is not particularly susceptible. I think it's difficult to say more than that, because the effect isn't always seen, and it can probably be mitigated.*
I've had plenty of rieslings under screwcap, all of which seem to have aged at a slow but non-glacial pace. I've had a few rieslings of significant age under screwcap, and they've been very nice indeed. For more than that, you'll need to wait for the incomplete science, I'm afraid.
* Screwcap liners, which are the actual closure, can be adjusted to provide specified levels of oxygen ingress, which would eliminate the reduction issue. The problem is that since we don't know exactly how much oxygen, if any, is "right" for any given wine to age as we expect, we have no idea what that adjustment should be.