Family member has 07 Suburban (essentially same as Tahoe) which gets used for local family chores. Since the driving they do is mostly urban/suburban, mileage is 12-13mpg consistently. No significant problems at 80K mileage. I've drove it one weekend and found it to be roomy/useful and a bit dull.
Now it's a thread on honesty, not that I give a flying f.
I find the IS350 to be plenty economical, not needing the dulled driving experience of Prius/Insight hybrid.
Tires are most likely the biggest contributor to observed difference in quietness.
I just had my 2007 IS350 value springs done, car was in shop for 2 days. I had a new loaner 2011 ES350 w/17" wheel/tire setup, very quiet.