Attached a new patch that hopefully addresses the concerns expressed
in this thread:
https://lists.gnu.org/archive/html/guix-devel/2015-02/msg00814.html

I do not create a ready to use bootstrap package because the resulting
store entry amounts to 940MB! I just keep an unmodified fixed version
tarball (68MB) and take care to patch it in custom phases of the final
GHC package. This is to avoid having to transfer almost 1GB of data
each time we need to rebuild GHC (due, e.g., to an update to a
dependency) and is much faster.

I took this route after suffering from extremely long and unreliable
transfers of the TeXLive package. (Yes, I tend to use --no-substitutes
for TeXLive, but when it needs to be transferred as a result of
dependencies things get trickier.).

Regards,
Fede