I've just tried to emerge dev-lang/mono 2.10.5 several times on hardened amd64 using Core i7-2600K@4600MHz without MAKEOPTS=-j1 limitation in ebuild - it works ok with my default -j9 and build time lowered from 683 seconds to 295 seconds. Looks like this hack with forced -j1 doesn't needed anymore.
I think upstream still considers parallel build a bit risky :-/
dropped in 3.0.7