Dont worry about the max rate your modem says, it is basing it usually using all your spare SNRM.
You need to go by your current SNRM. Skys system is set to 7db, which yours sitting at. I would look at your internal wiring, you should hit the full service on your line so something else is causing it to sync a bit low or you could just have poor copper but im going with bad internal wiring.
After your testing period, try connecting in the test socket on the bt faceplate.