As described here http://www.informationisbeautiful.net/visualizations/million... Facebook code base (~60 MLOC) if almost 4 times bigger than Linux 3.1 (a mere 15 MLOC).
Even worse - with next git gc you will loose your data as parts of graph with no pointer is assumed to be dead by git.
As described in http://mercurial.808500.n3.nabble.com/named-branches-vs-book...
> We have users with thousands of named branches in production and have > done tests on up to 10k branches and the performance impact is fairly > minimal.