Then I guess we will just have to up the ante to save-or-die spells, or spells which disable foes without saves.
Wizard moves, casts quickened true-strike and fires an empowered orb spell. Cleric moves and casts destruction. Druid moves and casts finger of death. Fighter moves and .... swings for 2d6+15.
Wizard's damage: about 76.
Cleric's damage: about 35 (monsters failing fort saves--pull the other one)
Druid's damage: about 26 (monsters failing fort saves--pull the other one)
Fighter's damage: 22 by your calculations.
But that fighter is not doing his job right. At level 16 or so (which is where the spell options you mention are not the absolute best thing in the characters arsenals) the fighter should be at about 25 strength (16 start, +3 level, +6 belt) with greater weapon specialization and melee mastery and a +1 holy, wounding greatsword made from starmetal. At that point, if he is attacking a high AC target where he dare not power attack, he's dealing 2d6+19 (12 str, +6 feat, +1 enhancement) +2d6 holy +1 point of con for an average of 31 points of damage and one point of con. If the cleric gave him a greater magic weapon spell, that's 2d6+22 +2d6 +1 con for an average of 36 damage and 1 con. If he power attacks for five (which he probably can without much trouble), that's an average of 46 damage and 1 con--which puts him pretty close to the wizard in effective damage inflicted.
Now, sure the monster probably have a 5-30% chance of failing those fort saves and dying outright.
But the wizard could also have dropped a quickened dimension step spell to put the fighter in range for a full attack, at which point said fighter clicks his boots of speed, and unloads five attacks for a probably damage around 180 and 5 points of con or more if he power attacked.
Now, give the fighter an "all your attacks are touch attacks" ability and he power attacks for 16 and still hits all five attacks for a total of 340 damage and 5 points of con. Are you seeing the broken yet?
I guess the problem here is more of the fighter's reliance on the full-attack action. He is capable of dishing out astonishing amounts of damage (and unlike a caster, can do so without needing to expend any resources), only problem is that he needs a full round to do so (which sorely hampers his mobility). The discrepancy in damage between a full attack and a single attack is just too great.
Is it any surprise that the (optimal) tactics of many high level monsters such as the balor, pit fiend, titan, molydeus, planetar and solar all involve spamming high lv SLAs while remaining on the move to deny the fighter classes their full attack, rather than bothering to actually wade into melee? Or at least, they would do so only after their spells have sufficiently softened the opposition.
Depends on the high level monster in question. When I played Age of Worms, our fighters were the ones trying to stay out of range of Dragotha's full attacks. (Of course, I was playing a fighter/scout with the PHB 2 improved spring attack line of feats so, in general there wasn't much difference between my full attack and my move+attack).
Considering that the belt of battle will likely also house an enhancement strength bonus (using MIC's revised pricing guidelines to combine belt of battle with belt of str and con), I doubt it...
Who do you think uses belts of battle? It's not for fighters; it's for wizards and clerics. Flame strike+Belt of battle+Flame strike. Prismatic wall+Belt of battle+Telekinesis.
Now, if you use the MIC's revised pricing guidelines, the fighters will probably have belts of battle too, but they'll either use them for extra move actions so that they can full attack in round 1 or blow them all at once for two full attacks in round 2.
Either way: belt of battle=broken. There's no call to bring 3.0 haste into 3.5.