On the old TDIs (non-PD ones) you used to be able to adjust the ignition timing to slightly advanced, which would
help improve mpg.
I'm not aware this is the case on the PDs, I think it sorts itself out electronically as there is no adjustment on the injection pump. It's a completely different design.
It sounds to me like whoever changed the belt last on your car was just a little careless, and is a tooth or so out. It is true they should be tight, minimal slack.