I doubt 270 mph has been exceeded for an airliner, but it is possible depending on the aircraft.
The worst case scenario would be a max weight takeoff followed by an immediate flap less landing.
The B747-800 has a max takeoff weight over 900,000 lbs. If one had to had to do a flap-less emergency landing shortly after a max weight takeoff (and unable to dump fuel), it would use Vref 30+40 (about 226 knots) for an approach speed, and that would be about 260 mph.
I don't have access to all the B747 speeds, but this chart does give an idea of the speeds needed for a flaps up landing.
Source: B-747-400 Flight Crew Training manual
