I'm not 100% sure but anecdotally it seems like players all get the same number of total development cycles, whether it's 15 or 20 or whatever. So if your IFA signs with 2 days left in the season as opposed to 2 days into the season, the guy at the end gets his extra development on the back-end in year 3 or 4 as if he was on normal schedule. This opinion could definitely be wrong, because like mktulu I'm not sure how you'd test for it.
I only have experience with 2 major players who signed after the minors were completely done (no minor league playoff games),
Louie Kondou (draft) and
Paco Trevino (IFA). Because they did not participate in minor league playoffs, each one carried over to the next season as Pro Years: 0, so I'm not sure whether that detail makes a difference or not. Maybe it did, maybe they got "extra" development in the first 2 cycles before the draft that other PY:0's did not have access to. In Kondou's case, he's still getting a ton of development at the very back end (Pro years: 4, ML years: 2) as opposed to the very front end (Pro years 0, initial minor league season). Only one example so obviously remains inconclusive