I'm not sure that interleaving affects the synch speed, it should only really affect ping times, IIRC.
I have interleaving on, and synch at 8128 which I think is the max for ADSLmax (no pun intended).
My SNRM is currently 10db and my attentuation is also 22db. Though to be honest, you're unlikely to see much change by synching that little bit higher. According to Kitz
the difference would only by 0.15Mbps
ZeN 8000 Pro
Draytek Vigor 2600g -> Vigor 2710n
Down Speed Synch: 8128000
Up Speed Synch: 448000
What I get: