No world regen required? Not true. This comes from experience. Upon adding and removing [SPEED:1] to and from creature_standard.txt I didn't see the change until after I created a new world.
Loading the existing save: no change
Abandoning fortress, starting a new game and reclaiming the last fortress: no change
After creating new world: saw it instantly, dwarves moved ridiculously fast
Removed the line one last time and loaded the game: they still moved ridic fast
As far as skill learning or decay rate goes, the changes I made seemed inconsistent even after creating a new world. I added the line [SKILL_RATES:5000:NONE:NONE:NONE] there (above the default dwarf PHYS_ATT and MENT_ATT stuff) and after mining about 500 tiles with a single dwarf he went from Not Miner to Talented. To me this seems it was somewhat faster but not nearly as fast skill improvement as I would've expected. I expected he'd be Legendary or something, going from 100 to 5000, that's 50x increase after all. But then again, I'm a newbie at DF.
Is it supposed to be SKILL_RATES or SKILL_RATE? Where do you get these... names. The magmawiki link seems iffy in this regard, it uses both terms (not to mention the other typos, may the author suffer 50 lashes).
EDIT: New world, changed SKILL_RATES to default 100, same amount of digging. McChosenDigger ended up Talented, no more, no less. So yeah, something's wrong here.