It's not too late! The wasm binary encoding is open to change up until the browsers ship a stable MVP implementation (then the plan is to freeze the encoding indefinitely at version 1).

The primary advantage of LEB128 is (as you mentioned) that it's a relatively common encoding. PrefixVarint is not an open source encoding IIUC.

We'll do some experiments in terms of speed. If the gains are significant we may be able to adopt something similar (this [0] looks like a related idea).

Thanks for the suggestion.

[0]: http://www.dlugosz.com/ZIP2/VLI.html