To address the question of what is the total input-output latency of that system one has to consider that this varies quite a lot between different setups. The overall latency might consist of:
* The scanning interval of the hardware controller buttons
* The latency of the hardware midi controller for MIDI Out
* The MIDI protocol latency
* Additional latency that will be introduced when transporting MIDI over USB.
* Additional host driver latency (if you use a computer as sound generator)
Although there are some sources out there like (http://www.soundonsound.com/sos/sep02/articles/pcmusician0902.asp) that tried to measure these individual factors, you better go for a measurement of the overall input-output latency. I recently watched a session about Android low delay audio from this years Google I/O
They actually had a nice setup to measure the overall latency. They've used a 2ch scope and put one channel on the output audio and the other to a self made circuit that measures light intensity of a hardware led on that device. In your case you could just put a pressure sensor (piezo) on one of the drum pads to measure the latency in a similar fashion.
I would not be surprised if most of these drum pads will have delays in the range of 3-6 ms. With MIDI there is very little change you can get into the sub ms range. This might be the reason why most drum triggers are still using analog transmission.
Dipl.-Ing. Fabian-Robert Stöter
International Audio Laboratories Erlangen (AudioLabs)
Universität Erlangen-Nürnberg & Fraunhofer IIS, Audio & Multimedia