The 20% firing speed penalty on the Tomislav also applies to the sound effect the Tomislav makes when firing. Don't remember exactly when this was added but this was not always the behavior.
So if you want to make a sound mod for firing the Tomislav be sure to speed up the track by about 1.09x (not sure why that exact number but it's the one that sounds the most right to me).

On a related note, the Tomislav has no file for a spin-up sound effect unlike all the other miniguns. Makes sense, but it means that the windup sound effect is technically still playing when you rev up the Tomislav. So if you change the windup sound effect to be any amount longer you're technically cheating.
(Not that this matters on valve servers, sv_pure 1 means weapon sound effect mods won't play)