Okay, put it this way: with a 20' reach, what you're really trying to do is use yoru AoOs, and your Knockdown (any spiked chain fighter with the name gets Improved Trip and Knockdown, heh), to shut the enemy down. As many as half yoru feats have been invested in achieving exactly that end: using AoOs to severely curb your opponent's ability to use THEIR "neat tricks" against you, without taking a couple extra hits in the meantime.
But DC 15? Past 11th level )or 21st level, if it's a CC skill), anyone that wants to be able to tumble, can GUARANTEE they will make the roll -- just by having max ranks in the skill.
I honestly think it should be MUCH harder to tumble past a Fighter(20)'s threatened area, than it is to do the same, past a Fighter(5).
The DC for Tumble to negate AoOs is simply way too easily made; it should be scalar to the level/HD/etc of the foe you're tumbling past (so, against foes of equal level to yourself, you should have the same rough chance of success, be you 5th, 10th, 20th, or 50th level).
One way to look at this: shouldn't it be easier to tumble past the mooks, than to tumble past the BBEG himself?