I tried to make RaspiDrums work with two sensors yesterday, and guess what, it worked (after a minor bug fix...). This a little step forward, but an important one that has now disappeared of my TODO list.
The good news is that the latency doesn't change at all. So, whatever the number of sensors I use, it should remain below 10 ms (tested on Raspi 2B only).
I guess the next step is to start thinking about a viable solution for the hi-hat. I already have an idea for the software, so I just need validate it, and to find the right sensor for the pedal.