filter-branch IO optimization

Hi folks,

for certain projects, I need to regularily run filter-branch on quite large repos (>10k commits), and that needs to be run multiple times, which takes several hours, so I'm looking for optimizations.

The main goal of this filtering is splitting out many modules from a large upstream repo into their own downstream repos. This process should be fully deterministic (IOW: running it twice at the same input, should produce exactly same output, so commit IDs stay the same after subsequent runs)

My current approach is most likely yet a bit too naive:

#1: forkoff new branch from current upstream
#2: run a tree-filter which:
    * removes all files not belonging to the wanted module
    * move the module directory under another subdir (./addons/)
    * fix author/comitter name/email if empty (because otherwise fails)
    * fix charater sets and indentions of source files
#3: loop through `git filter-branch --prune-empty` to get rid of empty
    merge nodes (which otherwise remain really a lot), until branch
    remains unchanged
#4: run plain rebase onto initial commit to linearize the history

All that is done is on per-module basis (for now only about 10, but soon can become much more).

One thing I haven't tried yet is using the -d option to move the .git-rewrite dir to an tmpfs (have to clarify some operating considerations first) ;-o

The next step I have in mind is using --subdirectory-filter, but open questsions are:

* does it suffer from the same problems w/ empty username/email like --tree-filter ?
** if yes: what can I do about it (have an additional pass for fixing that before
   running the --tree-filter ?
* can I somehow teach the --subdirectory filter to place the result under some
  somedir instead of directly to root ?
* can I use --tree-filter in combination with --subdireectory-filter ? 
  which one is executed first ?

thanks