Looks like about 10% performance hit (presumably a combination of extra bounds checks and larger data structures), and about 10% code size increase. The size increase probably warrants more investigation since I wouldn't expect the larger pointer sizes to show up very much in the binary (just the static data structures of which I doubt there are that many).
I got some initial performance numbers by running the emscripten benchmark suite under v8:
Looks like about 10% performance hit (presumably a combination of extra bounds checks and larger data structures), and about 10% code size increase. The size increase probably warrants more investigation since I wouldn't expect the larger pointer sizes to show up very much in the binary (just the static data structures of which I doubt there are that many).