The performance of the final result is actually pretty good, many times faster than JSON.Parse for arrays of floating point data. Here is the data:
http://jsperf.com/json-vs-base64
The ThreeJS pull request to adopt this into the core library is here:
https://github.com/mrdoob/three.js/pull/3420
I think the applicability of this for speeding up JSON is quite wide, much wider than just ThreeJS.