It's actually an extremely appropriate cap, nowhere near awful.
Temple of Elemental Evil is based off the original D&D adventure, and for the most part it stays true to the original written adventure for 3rd edition D&D.
Circle of the Eight only increases the cap to 20, which is the standard cap for most any regular D&D adventure.
You won't find a mod for this that increases it to 99 or something like most Action RPG's, it's not the way the game is played.
[color="DeepSkyBlue"]Listen up maggots, Mr. Popo's 'bout to teach you the pecking order.
It goes you, the dirt, the worms inside of the dirt, Popo's stool, Kami, then Popo.
~Mr. Popo, Dragonball Z Abridged[/color]