Are you aware of any comparisons that shows "truffle appears to generalise to other languages a lot better"? I'd be interested in reading them. :)
The performance of various RPython interpreters like PyPy and HippyVM are pretty impressive.
Not aware of any direct comparisons. Perhaps I am engaging in hyperbole :) The reason I said that is I thought RPython is mostly for speeding up Python, and wasn't aware of HippyVM. There are tutorials on how to use Truffle for other languages and some apparently useful implementations though.
RPython is definitely a Python-first thing. PyPy is the only VM written in it that's actually production-ready at the moment. I'm guessing that has more to do with effort invested than any real limitations on RPython's part though. I'll definitely read up more on Truffle. :)
I don't know if it could or couldn't be done well in RPython, but one application of Truffle that shows off how well it generalises to other languages is a C interpreter. We use it to interpret Ruby C extensions.