"A few boxes" of anything isn't worth it. Standardization is critical to having any sort of profitability. As soon as you start spinning up a small number of custom boxes, you're adding a lot of work. Enough work that the "premium" you mention would be pretty large (large enough that it would probably wipe out any price advantage in the first place).
If you really need super fast i/o, then why are you using a VPS in the first place? Virtualization adds a non-trivial amount of overhead to i/o, and there's really no way around that (technologies like VT-d and virtio are helping, but virtualization is still virtualization)
