Early on I followed the directions on kits, however lately (last year or more) I have deviated from specifically following them.
I have found that racking to a carboy anywhere around 1.010 or lower, extends the fermentation time quite considerably in comparison to just snapping the lid down. Usually when I am down to 1.010 I snap the lid down and within a few days (no more than 5) I am down to .0996.
Then I rack to a carboy, degass, clarify, sorbate, k-meta, etc…etc….
My thought has been this-WE and many of the other kit manufactures will tell you that having airspace in the carboy during the clarifying stage is acceptable. While I do not necessarily buy that train of thought, I will believe it when the must is still throwing gas.
Then again when I am making a kit, I have 6 gallons of must in a 6.5 (maybe 7) gallon bucket, there is not that much room. However I would leave just 1 or 3 gallon of must to ferment out in a 6.5 bucket. I would rack that.
I have completely fermented in the for my last 7 kits and they all taste fine (actually I have not tasted my last 2 kits, since they are super young, but we can presume there will be no issue).