More jumping the gun IMO. Skipping steps for the sake of brevity, only to have to go back and spend lots of time elaborating, is a specialty of mine.
To go over the stuff I skipped:
1 - Excessive metamagic reduction creates excessive damage (and/or excessive riders on top of the damage). Your post details it, as does googling the Mailman sorcerer.
2 - If your group's op level is okay with that sort of damage (i.e. one-shotting things several CR above the party level) then all is well. No bans or changes needed.
3 - If it is not, then you need to tone things down. Your initial post suggested banning Arcane Thesis and its ilk . . . which works, but due to the difficulty of utilizing metamagic in E6, keeping it around in a non (or less) abusable form is worthwhile, hence my suggested alternatives.
Sorry for the confusion!