The game doesn't use any measurement in particular; it is in no way properly scaled to anything in the real universe. Just look at how big the ships are to the size of the planets (or the planets to the stars), that should show you the scale is totally arbitrary. 
By convention, players typically use "Meters" when describing things like weapon and ability range, and in fact it might even be called that in game somewhere, but this really cannot be expanded for things like gravity well/phase lane length since that would mean planets in the Sins universe would have a radius of less than 100km and the gravity well sizes would be measured in 100s instead of 1000s of km.
That said, if you do want to change the speeds, I would adjust the numbers in the gameplay.constants file, as that applies to everyone and you can have a variety of speed settings that way. Also just in case you're new to modding, NEVER change the base files in your install directory, always copy files over into a new mod.
