I had a Mk VI Golf TDI. Real world under similar conditions that you quote, was about 5.0 L/100 km average (in summer). My 2016 Golf TSI, real world, 6-6.1 L/100 km (also summer). On the highway, 5.6 with the TSI, 4.6 with the TDI. So factor in about 1 L/100 km more for the TSI. Right now RUG is a few cents cheaper than diesel where I live (near the US border, so we buy our gas in Sutton which is a few cents cheaper than elsewhere) and all winter diesel was considerably more expensive than RUG, so I expect the difference in operating cost would be minimal though I haven't tracked it.
You're correct, the fuel capacity on the TSI is 5 L less (at least on the Golf, not sure about the Jetta). If you were doing a straight highway drive at 100 km/h you MIGHT squeeze 900 out of a tank, 800 for sure. 750 tops in "normal" driving, but I usually end up filling at around 600-700 simply out of convenience and the desire to start any trip over 100 km on a full tank. However since I'm retired now I don't mind so much, my driving has gone from nearly 45k a year to 22k a year (which is a bit above average but that's the problem with living in a rural area, you have to drive just to pick up a litre of milk... a walk "around the block" for me is 8 km)
The Golf has the 1.8 TSI in all models, while the Jetta has the 1.4 in the lower trims. I suspect the 1.4 would do maybe 0.2-0.4 better, but I have no real world experience with it.
Keep in mind I drive for efficiency, that is, cruise at 105 km/h (same speed as the trucks, so minimizes passing/being passed by trucks), lots of coasting to stops, anticipating lights, and absolutely NO tailgating, and trying to keep a large gap in front in heavy traffic to minimize braking. I did that with my TDI as well, with the result above. Add about 0.8-1.0 L/100 km in winter depending on weather.
So far I'm extremely pleased with the 1.8 TSI. Quieter, smoother and quicker than a TDI, and reasonably efficient for a gasser. Like a TDI, it is easy to beat the official estimates with this engine.