No, technically the other factors increase the overall EL of the situation. The CR of the Tarrasque does not change unless it gets Templated, extra HDs etc.
My point was that the Tarrasque is a powerful and deadly monster and at it's CR is worth it, you just have to make the characters fight it on DM terms, not Player fantasy land theoretical optimization terms. Yes, a low level Wizard can kill the Tarrasque. In theory. In practice though.... it should not happen.
And the Tarrasques CR is fine where it is. I'm sure anyone can pull off a one in a million theoretical kill, with a lot of stipulations like "Oh and I need 500000 more GP than I should have at this level"; ask that Fighter, Warblade, Hexblade, pretty much every class that relies on melee, if the Tarrasqe is CR 20 and for the most part they would say "No, it should be CR 30, it's impossible"; so yeah, woopity doo. You can use player knowledge (cheat, essentially) to beat a monster. Good Job.