That year tdi is a unit injector, meaning it is a common rail and does not have an injection pump to be kept in time. My tdi only ever got 39-42 mpg, I find 50 a bit high, but driving conditions and driving environments have a lot to do with economy.
The set up of that engine is hard to be a tooth off, if that were the case it would run like crap from the first start up after the timing belt was installed.
A few items to check…
are all the sensors that were removed plugged back in, I am assuming yes since there are no indicator lights illuminated on the dash.
are all of the intake hose claps on and tight, the loss of boost from the turbo will cause low fuel economy and lack of power. (I myself believe this could be the problem, I have seen it and I have done it, it is easy to do and only becomes a problem under boost (engine load) conditions!)
Have you changed your driving habits? ( Using a/c now? new job with a different route to work? find a different fuel station that you have not used before? installing fuel additives?)
I am not sure how the cam gear is attached to the cam, (splined, keyed, tapered, etc…) but it is possible that the pulley slid on the gears, but under normal timing belt installations this should never have to be removed, or loosened, and if it is, it is a pain requiring special tools to get it back true. If this is suspect to be the problem, I would ask the mechanics that preformed the work if they may have done this, and look for the options on how to fix it.