Certifications are just another way to bloat the REQ system. The drop rates only change slightly until you have unlocked all certifications. But, consider that certifications for better items are rare in themselves. So, take a Plasma Pistol and Phaeton for example. While a player might unlock a certification for the PP early on, he/she will end up getting more PPs than before but still not many Phaetons. That in turn creates the need for more packs to unlock the Phaeton certification to get the drop rates more balanced. So in the end, certifications don’t mean anything since the end result after acquiring all of them is the same set of drop rates as not having any.
That said, I don’t have the actual numbers, but I doubt 343i would “rebalance” the drop rates for players who have a few or no certifications vs. those who have all of them. So, to answer your question, I believe the numbers are fluid (albeit, with only slight variation) until you unlock all certifications.