1) Why does the bandpass filter only have very small impact?
2) What is the maximum Q-factor for bandpass filter?
The Maths Channel bandpass filter is effectively a single pole filter, so you only get a gentle roll off of between 3dB and 6dB/octave (there is no resonance). So, there will be a practical limit to how small you can set the bandwidth of the bandpass filter, before the filtering effects start cancelling each other out, and the signal level starts to reduce.
You may be able to use a trick or two to make the slope a bit steeper, but ultimately the Math Channels don't provide a full digital filtering toolkit for amplifying narrow bandwidth signals, with surgical precision. However, they do provide you with, a very effective means to clean up and condition the signal for analysis, when complimented by the other software tools at your disposal, and then give you the capability to transform the signal in a variety of different ways, some of which even make ingenious use of unintentional ways that the Math Channels are implemented (as can be seen from some of the posters on our forum).