If 280 was the absolute minimum speed they wanted you to penetrate turbulence at, then they wouldn't publish the following in the factory manuals:
Flight Crew Operating Manual, Volume 2
Limitations
Page 02-06-04
8. TURBULENCE PENETRATION SPEED
Maximum (emphasis added) air speed for turbulence penetration is 280 KIAS or 0.75 Mach, whichever is lower
At ACA/Independence we used 280/.75 as max penetration or 250/.70 for passenger comfort.
Also, FWIW, design maneuvering speeds vary depending on the aircraft weight and pressure altitude from as low as 161 knots (sea level, light weight) to as high as 305 knots (30,000' PA, 53,000lbs).
Design maneuvering speed (Va) is defined as Full Application of rudder and aileron controls as well as maneuvers that involve angles of attack near the stall, must be confined to speeds below Va.