Hi Timothy,
As Leszek points out, I have some plans for faster BigInt algorithms in V8, and some standalone code that I haven't spent the time yet to land upstream. An important contribution you could make to that project (especially in order to accelerate it) would be to describe a use case where this matters -- we like to spend our time (and binary size) on things that users care about, and so far we haven't heard about anyone needing to multiply/divide BigInts with several thousand bits. Do you have a specific scenario in mind that would benefit from this, or are you viewing this mostly from a theoretical perspective ("because we could")?
That said, of course I wouldn't want to discourage you from contributing code to V8! Are there any other areas you might be interested in?
Cheers,
Jakob