Method::Signatures has come a long way recently due to @barefootcoder's work. I'm happy enough with it that it doesn't seem worth the trouble to rewrite it in perl5i.
Before it can be introduced to perl5i, a few things need to be done.
Method::Signatures is a good choice. We already use Devel::Declare. If we're going to do stuff with OO, it's probably going to be with Mouse. Method::Signatures is 80% of a Mouse equivalent to MooseX::Declare.
What about Method::Signatures::Simple? My gripe with Method::Signatures is that is introduces a dependency on Devel::Declare which in turn won't compile with MSWin32 (Strawberry Perl 5.14.2).