Has been suggested a million times, always turned down.
Site load is more important here, a 50 message inbox should be adequate for any normal user.
This right is, and should be reserved to M++ and up.

Site load is a big concern since like you said, this is a growing site and we don't want it to get any slower.
I'll take a fast running site over an augmented inbox any day.

No from me.

i like that idea

It isn't that much of an effort to clean it!

Voted No, sorry

Please stop bumping this suggestion guys. It isn't going to happen.