Heres a graph i did roughly (i can't draw
)
The graph represents Christians graph with the BCF filter.
Grey is filter 1, maron is filter 2.
Filter 2 is designed to eliminate all the IR's. Lets predict that the maron line goes down at 750 then goes flat all the way pass 1200nm but looking from 300 - 400nm range, it doesn't block any UV's just mainly IRs.
Filter 1 blocks the UV range (300 - 400nm) then goes up to cater for visual light including H-Alpha, then it goes back down again at 700nm. why would canon put 2 filters which does the same thing? To me, filter 1 doesn't block IR just the UV whereas filter 2 doesn't block UV but blocks the IR.
Filter 2 also blocks the H-Alpha range therefore changing filter 2 is more ideal than changing filter 1.
Therefore with my cam, taking filter 2 out will unleash the IR power but adding the BCF filter will slightly cuts some IR signal.