I'm a running a jts in 156 Sele format atm, having run a 156 GTA for a year previously. My GTA would realistically get about 24mpg combined. The sort of driving i did (city and fast generally) this ended up being in the teens. The JTS gives me over 40mpg on a run using high octane fuel (it likes it) while i get low 30s combined. Pretty good on the old fuel.
All i would say is that the JTS likes a service, warmer weather and high octance fuels. These things will make all the difference.
One of the main reasons I cannot part with the JTS (which was meant to be temporary) for the moment is the fuel economy. I'm also enjoying the sele box, its lovely.
Previous Alfas: SudSprint 1.7, 156 2.0ts , GT 1.9 JTD, Alfa 156 GTA, Alfa 156 2.0JTS Sele