The Ka-50 flight manual from DCS says weathervaning starts noticeably appearing around 90-100 km/h, and to let the aircraft turn itself around if you reach those speeds tail first. It specifically states that as the maximum speed for tail first flight without altitude loss, and has some poorly elaborated bits about the nose tilting down during the induced turn.
In short, I'd say a 180 degree turn at that speed might be a bit much, but it could be good starting point for turn limits.
I am assuming the Mi-48 is quite a bit less aerodynamic than the Ka-50, so turn limits would probably be lower.

For max speeds, the Ka-50 has it around 300, going lower with altitude and loadout weight. We don't really have to worry about altitude for now, since Arma 3 takes place at sea level, but co-axial configurations have a pretty sharp speed limit since the upper and lower rotors collide if you go too fast or make harsh manoeuvres close to max speed.
I had expected the Ka-52 to have lower max speed due to higher weight, but I can see a great many things affecting that, like rotor spacing and lift from the aircraft body.
I am unsure if we should go with those limits, since I don't think the more gradual problems at speed (dissymetry of lift and retreating blade stall) for single rotors are simulated.

Would you mind giving sources for the speed figures you have? I only really have the flight manual to compare to, and those figures probably have safety margins added.