It's always been like that, it's actually gotten worse since G.INP was implemented. Potential sync was normally around 31Mb and my real sync was always around 26Mb. Then a good while ago my actual sync got stuck at exactly 20Mb (20000 on the dot) as if it was being artificially limited. After a few months it finally fixed itself but I've never exceeded 24999Kb (2.67MB/s actual download speed) since then. It's really annoying.
You mentioned fastpath. This line is heavily interleaved (or at least it used to be), does G.INP have its own fastpath/interleaving vs the normal part of the line or are you saying that this line doesn't appear to be interleaved at all anymore?