Are you still using the stock knock balancing rate values in table B1940 and is the knock only above 5200rpm? If so I think there is some false knock occurring based on my test results. I ran out my tank of 93 octane (your 98 RON and filled up with 89 (your 92 RON) and I actually saw slightly higher final timing on the same amount of meth/water injected.
14s should definitely be possible- I was running the stock transmission calibration when I ran the 15.2 at 21psi boost, but had the ported TB and ported intake manifold installed and I still need to put those parts back on the car. I see ~7% higher MAF readings at the same boost pressure with the ported TB and intake manifold