The Etendard formula should inherently be quite suitable for a supersonic fighter, being optimized for low transonic drag. The prototype Etd IV is said to have exceeded Mach 1 straight and level and the IVM was capable of Mach 1.3 in a dive.I suspect that [a] the Etendard formula simply reached the end of its development potential with the ATAR 8 and was never intended or engineered to be a supersonic fighter.
Keep in mind it’s the same wing formula as the Super Mystère (which could fly Mach 1.3-1.4 with the Atar 9B - aka. SMB4), but further optimized with reduced thickness for less transonic drag. And of course 20% smaller total wing area which also reduces drag.
Likewise the Etendard’s narrow, area-ruled fuselage should be much less draggy than the Super Mystère.
Overall it was superficially similar to the F11F which as we know did quite well once fitted with a good afterburning engine (the J79). The Etendard would never be as good for the high-altitude interceptor role as a pure delta like the Mirage III, which would always win in terms of low drag & structural weight. But as a Mach ~1.5+ (my guess) tactical fighter optimized for high maneuverability and good runway requirements it seems like it could have been a more balanced design.