Modded "Olga-24" with 1600 kW was accelerating hard until it suddenly stopped at exactly 320 km/h.

Test with a second "Olga-24" with 2800 kW also stopped at exactly 320 km/h.

Is there some mechanism preventing speeds higher than this?

If so, it should be removed or increased. It seems you cannot even achieve 200 mph (326 km/h) currently, many supercars now have top speeds in excess of 400 km/h. Some of these supercars are even already on Steam Workshop but cannot achieve anywhere near their real top speeds in DayZ.

Example, acceleration stops at 320 km/h: https://www.youtube.com/watch?v=TpNfDaTzO_Y