What's the max 'g' that can be sustained by a "top fighter pilot"?
Good question! And as unlikely to get an honest answer as "How long is a top fighter pilot's willy"?
Being a long way past boasting on either score, I think I can recall that on the Meatbox, where we did not have 'g' suits but we did have a "upper" rudder pedals to get your feet as high as possible, descending in a spiral to maintain max permissible Mach/IAS, a chap who was in practice (v. important because special abdominal muscles are used) and who had not had too heavy a night before, could sustain 6 'g' whilst still retaining sufficient vision and mental capacity to function. Instantaneous 'g' could be much higher - as long as you did not pull the tail off!
Negative 'g' was not an issue. I know of know tactical manoeuvres that require it, and airframe stress limits are much lower in negative 'g'.
