Much as I love my DP2 Merril, I just don't see the Foveon sensor competing in high iso situations. Doesn't some of the problem arise from the fact that the lower layers need more amplification because of receiving less light? I'm no engineer, but that explanation makes sense to me, which leads me to think the issue will be solved by an electronics company rather than an optical company. Between 100 and 400 iso, though, my E-M5 just can't compete in terms of resolution. On the other hand it can do anything I ask of it rather well, which is not true of the Sigma.
A niche camera? Decidedly, but what a wonderful niche! I just doubt that the Foveon technology can be made to compete head to head on all fronts with good, general use Bayer sensor units. And I'm not convinced that the problem is Sigma, but maybe, rather, the sensor structure. I'd loved to be proved wrong, however.
Meanwhile, though it may be a minority opinion, I love the simple, straight-forward controls of the DP Sigmas. I haven't met a camera whose interface pleased me more.