A Blog About Film Photography and Film Cameras: Traditional, Experimental, and Alternative Processes.
The 2 different filters will make the difference in the contrast response of the films. SFX looks almost like T-max and a #2 yellow. However, IR filters will generally greatly increase contrast over others, even a dark red.
Yes I had noticed the difference in contrast. I think actually the darkest red filter with T-MAX get pretty close to the IR filter. More testing on it's way.