This is quoted from a Breaking Defense article published April 05, 2019
Speed: The Army’s minimum acceptable cruise speed, the threshold requirement, is 250 knots (288 mph); its preference, the objective requirement, is 280 (322 mph, incidentally the
intended cruise speed of Bell’s V-280). The Marines’
threshold is 275 knots (316 mph), almost as high as the Army’s objective; their objective is 295 (339). And that actually understates the difference, because the Army only asks for this performance at maximum continuous power — the highest the engine can sustain over a long flight — while the Marines want it at 90 percent of maximum continuous power.
The Marines have even higher speed requirements for brief sprints, something the Army doesn’t address".
Fastest level flight speed so far for SB>1 is 230 knots (264.7mph); for V-280 it's 305 knots (351mph).