One thing missing from this thread so far is the airfoil profile. The speed at which Mcrit occurs can be increased or at least the severity of the shock wave reduced by choosing a supercritical profile. Developed by Whitcomb at Naca along with area rule. If you compare the average sweep angle of new airliners to those designed in the 50's and 60's you can see the tendency towards less sweep. This is due to the use and development of supercritical Airfoils which also have a large internal volume.
So if you wanted a straight wing to fly at high subsonic speeds it would need to be as thin as your internal volume requirements would allow and use a supercritical airfoil.
The Saab 2000 shoots along and in the real world I would say thats about as fast as you will get without some new technology being introduced.