Also went back and checked the old Any% speedruns. There's a discrepancy there too, but less than what I saw in speedrun mode. Timing from the end of the "item unlocked" fade out until the next level's name appears on screen was 0.2-0.3 seconds faster per level for me than for redmars.

As for in-game time vs real time-- it seems like load times are relevant in this game. I just compared my run of speedrun mode vs. redmars' run, and at the end of the run there was a ~10 second discrepancy between in-game time and real time. As far as I could tell neither of us paused. I timed a couple of level loads and found that mine were 0.5-1.0 seconds faster per level.

Clipping through walls w/Input Display:

This only works against big, flat walls. Can't have any props in the way, and some walls just don't work for various reasons.