I'm comparing Breras rather than the 159, but my 2.2 JTS returned 28mpg (measured over 2,000 miles) from my typical driving conditions and my 1750 TBi is a tad over 30mpg over a similar measurement distance.
I drive on cruise control at 70 mph on motorways and uninspiring A roads (like the A30 hereabouts) but really indulge myself in speed on country roads.
I didn't mod the 2.2. The 1750 is a far better engine to me, lots of torque, really responsive and beautifully smooth. I'd hoped to get a bit better consumption out of it, but the other advantages are myriad.
I believe what doesn't kill you makes you...stranger.