A dropin means an emitter assembly that can be easily swapped in and out of a light. Many SureFire lights and MagLites can take dropins to replace the stock bulb.
However, the TK11 has a sealed head. There is no dropin for the TK11 because the emitter assembly cannot be removed. Though it may be possible to swap the R2 emitter for an MC-E to get more lumens at the expense of beam quality, such a mod would have to be done by someone who is good at disassembling lights and mounting/soldering small objects.
Short answer: You're pretty much stuck with what you've got...