Reading through the actual description in the DMG, I see no problem with this at all.
- Red Wizards are a paranoid lot.
- There is only a limited number of available "Circle bonus levels" with which to apply metamagic.
- Metamagic applied at the time of the Circle, not on the fly.
Given these facts, the Red Wizard in question will probably be more worried about trying to discern who actually put these chump "Heroes" up to this act of utter stupidity, and what his enemies hope to gain in the process, than actually pouring all his resources into one PC-slaughter feast. Because, the Red Wizard assumes that is what his opponents are trying to trick him into doing. Then his so called "apprentices" will be lurking in the wings to finish of their "mentor".
Given 15th Red Wizard, with 9 9th level "apprentices", sure he can access an incredible 45 Circle bonus levels. So he expends 25 of these to increase his caster level to 40. Which means he is likely to overcome any Spell Resistance he might face. 20 Circle bonus levels left to apply metamagic feats (Heighten, Empower or Maximize) to any spells he has memorized at the time of the Circle.
Given the multitude of threats a Red Wizard may face in anyone day, would he choose to raise one or two spells out the wazoo, or would he spread them over a multitude of spells, increasing his versatility and staying power? Personally (if I was a Red Wizard), I'd rather choose the later, rather than count on my "friends" to back me up should a "big" spell fizzle for what ever reason.
Any idiot or gang of "heroes" trying to take on a Red Wizard at home had better come well prepared.
But there are far more easier ways to defeat megalomaniacal paranoid wizards