To add to what edstainless has mentioned regarding strain hardening, I would expect that the 6% Mn steel would have improved fatigue strength. There are very general correlations between tensile strength and fatigue strength, where an increase in tensile strength results in an increase in fatigue strength. Also, with the increased ability and capacity for stain hardening, I would expect this would tend to reduce the tendency for fatigue crack initiation, which also would imply increased fatigue strength.
Regarding welding, Mn is an austenite stabilizer, and as such, I would expect this material to be welded with minimum preheat in the field. This material should be readily weldable.