Depends on whether you like to fly at 75% power, or save a considerable amount of fuel flying a few percent slower
A TB20 will do 155kt TAS at 10,000ft or so, at under 11GPH. It's a 4-seater but like most planes can't carry 4 present-day people and full fuel.
If you genuinely want to carry six then you need a
big plane.
Then you've got to think of oxygen, "loo privacy" on longer flights (hard to find 6 people willing to share), how much stuff they bring with them... what type of mission is this for?