What kind of battery do you use? I would suggest feeding the TK20 with low self discharge nihm. Buy some sanyo eneloop batteries and a charger. You should see a marked increase in performance compared to alikaline or regular nihm batteries (unless you "excercise" those nimh batteries regulary).
With CR123's you can certainly get brighter lights in a very small package, but I think it will be very though getting something small that actually will throw longer than your TK20. You need a pretty big head for that. On the other hand you will get a more "floody" beam and basically lit up everything between yourself and as as that light will reach. If powerful enough, this light may or may not reach as far as the TK20, and this may or may not work better for you than the TK20. The only real way of finding out is try it in practice.
Another factor is the color of the light. In TK20, the LED is designed with a warmer color that most people seem to prefer in natural environments. This comes at a price - those LEDs are not as efficient as "colder" leds. So you have a choice between slightly less light with a "warmer" color and a far reaching hot spot (TK20) or slightly more light with a "colder" color and larger, but not as far-reaching hot spot (almost every small other quality light). Some people at CPF claim that the far-reaching hot spot and nice colors of the TK20 makes it a better choice for outdoor use than more powerfull "cold color" lights.
Also, consider the fenix LD10 (1 AA battery) or LD20 (2*AA). They are much thinner and lighter, which makes them much more comfortable to carry in a pocket.That way you can keep all your lights on the same battery plattform (AA). They are certainly bright enough for almost every task you can put a light to (except lit up things beyond 100 yards), but of course the same brightness or more comes in even smaller packages with CR123s . . .
Please do post your decision and your experience here! I'm going the other direction (I am considering buying TK20), and would very much like to know your experiences. :wave: