Reduced drag on the P-39Q which had a pod/gondola mounted .50 under each wing. In this configuration (and possibly with a 4-blade prop of later Q models) I belive this was the fastest production Airacobra approaching 400 mph at ~10-11,000 ft.
I'm not sure about removing the 4x .30's internal mounted in the wings on earlier models.
Those speed figures may be exaggerated though as the P-40F seems much too fast compared to other figures I've seen (though Curtiss testing figures are reputed to be exaggerated). The first listed P-40F figure seems realistic, but second one with the short dashed line at 8,500 lbs seems a bit fast with an unusually high critical altitude. (with 380 mph at 22,500 ft)
That also seems high for the P-39D, for the N/Q (and some M's) this seems realistic, but not for the P-39D.