Sorry but some troubles with metric conversion :)
In real driving , Always with AC on, I do about 15-16 km/l .
Trying to have best mpg, I did at about 90m km/h (Always with AC on), more than 19 km/l, with cruise on.
In sport mode, with no care for fuel (and speed limits), 12.5 km/l ; with GS 450h I made 10.5 (more or less).
Highway driving , respecting limits and intense traffic, 15 km/l .
GS 300h is NOT slow indeed, even if I prefer to drive in Normal mode because in ECO mode the accelerator response is a bit sleepy, while if you need a better response SPORT mode is quite better and somewhat sporty; I tried with Torque android app a 0-100 km/h (0-63 mph) and while in ECO is 9.2 (as written on specs), in Normal is 9 and in SPORT 8.5.
A note, mpg could be better with smaller tyres, mine ones are 245/40 R 19
The dynamic behaviour is wonderful , in my GS also AVS, but it seems it's very good also without.
The max speed at 190 km/h is due to technical limits of eCVT, not a power problem; same issue was solved in GS 450h with a dual gear to achieve max speed limited to 250 km/h.