Oh, we're talking TO? What "well built Gish" beats a D2 Crusader at melee damage? What "well built Gish" beats a Hulking Hurler? There's a reason I picked that as my example, bud. Optimized Hulking Hurler is going to outdamage almost anything in the game. 5th level spells ain't gonna beat that man. And with the ways for melee to pick up "gishing" you're like at 5th to 6th level spells tops.
Edit: A "Well Built" Gish is solidly Medium Tier PO. There's nothing wrong with that, you could play that at almost any table, but you're not doing infinite damage. Or an arbitrarily high number of damage.