Hi Rick and welcome to the club. I've had a Passat 1.9TDI in a past life and the economy is excellent, you won't get anything like from the IS, but you're not comparing apples with apples.
I can't comment on full tank mileage, but can give you some indication of mpg to expect. BTW, my IS convertible is a little heavier than the saloon so you should better my figures.
My commute is 5 miles each way on a mixture of 30mph and 40mph free flowing roads and in the Winter I'm getting about 25mpg, that's with 2 cold starts of course. I used to visit Guildford frequently, mostly on the M4 with cruise set about 75mph and I'd often achieve 38mpg. Just this week I've been up to Chester and back, about 300 miles round trip up through the centre of Wales and it returned 33.1mpg on the tank fill, some very spirited driving on that run, occasionally up to 5,000rpm when overtaking.. My long term indicated mpg is 31.1.
All of these figures are generated by the onboard computer which I believe is a little optimistic. Hope this helps.