I'd echo the 330i sentiments....
I don't know about the 325i mpg. But my 330 has over the the last 3000 miles of about 70% motorway returned 30.7mpg. That's from an N52. If you went with the newer engine one guesses you'd be looking at more.
Even on my recent road trip to Wales around the Evo triangle and encompassing about 200 miles of spirited driving I got 28 out of her.
But then as my dad said you don't buy a 6 pot BMW then worry about the fuel economy...
