T-Glase really shows any kind of flaws in the surface. I chose a large layer height, because I had read that larger layers make the T-Glase surface more transparent, while thinner layers add opacity. (Is a 0.4 mm layer height possible with the Ultimaker²?)
Anyway, after experimenting for a couple of days, I produced a very nice, water-tight print of my cup! I had to set my filament diameter to 2.50 mm (even thought it's actually 2.85) to make the bottom water-tight. The final settings I used are as follows:
Layer height: 0.3 mm
Shell thickness: 0.6 mm
Bottom thickness: 1.2 mm
Print speed: 10 mm/s
Hot end temp: 220
Bed temp: 70
Filament diameter: 2.50 mm
You can see the z-scar in the second photo, but a person picking up the cup just to have a look would really have to scrutinize it to notice it.
I must say, T-Glase produces a very cool looking surface! Depending on the viewing angle, it can look glassy, metallic, or plasticky.