What options exist for creating a character focused on doing solely non-lethal damage? Are there ways to have increased damage output compared to a lethal-damage-dealing characters by accepting the restriction of being only able to do non-lethal damage as a trade-off? Can a character that only deals non-lethal damage be built as a "high damage" character, relatively speaking?
It's actually quite simple to do this. In the SRD, you'll see a rule that allows you to deal nonlethal damage with any weapon that normally deals lethal damage:
... which makes sense. You have to take much more care not to kill someone with a sword and just knock them out. But the option is there for you! And it makes for very cool character fluff, in my opinion.
Also, as @GMJoe kindly pointed out, there is a core enchantment in the 3.5 Dungeon Master's Guide (pg. 225) called Merciful for only a +1 bonus. You'll certainly want this (assuming you don't use one of the weapons which already deals nonlethal damage):
Other Source Books
The best sources for nonlethal damage in books outside of core come from the Book of Exalted Deeds:
Optimal Build (?)
So, really, it looks like your best bet would be to become a traditional, overpowered spellcaster, take the Nonlethal Substitution feat (which will allow you to use your spells to deal nonlethal damage), then use the various Vow feats to increase your other statistics. That looks like maximum leverage to me. I don't know of any other source books that would really contribute much to this effort.
As far as dealing more damage than a normal damage dealer, that is likely next to impossible without being very, very creative. For starters, there just aren't that many sources of nonlethal damage in the core rules. If you look in the Environment section, it might be possible for you to make some homebrew rules with your DM based on these (i.e., cold effects which numb instead of kill). If you want to stay strictly within the RAW, though, I think your best bet for having a highly functioning nonlethal character would be to have a spellcaster of some kind who only utilizes sleep spells, hold person spells, and the like. I didn't include this in the main answer because these effects don't technically deal damage, but it's an option which may appeal to you.