Sorry for double posting:
In vanilla UFO, the fire speed options also change the speed of the "hit-splash" animation (the animation that occurs after a projectile hits something); In OpenXcom, no matter the what speed of projectile is set on, the speed of hit-splash is still the same (Slow IMO), and the only way I found to make it slightly faster is to set the FPS limit to 0 (no limit).
And the same slowness applies to melee, explosions and psi-attacks.
And lastly, a question: You guys said that you used DOSBox as a reference for recreating vanilla speed, but what settings did you use? My own personal sweet spot was at around 40000 and 50000 set cycles for UFO and TFTD, respectively. Sure, some things became way too fast, but it was better than the frame-skipping that happened for cycles below those.