The F-8 was in the end almost as versatile as the F-4 and used accordingly...it just couldn't carry the same payload.
1.)A well trained GiB won't help much if the driver is quite inexperienced...I know that this setup is used when you start training new pilots, but still, once you at SHTF moment there isn't much he could do.
2.) Don't think of the F8U-3 as an uprated version of the F8U-1/2. It was a whole different aircraft, designed for a totally different purpose. And, despite its projected performance, an inferior aircraft compared to the F-4. That of course my armchair pilot's opinion, but I still think that it's quite valid.
Besides, even if, I don't think that it would have been that easy to add a gun or Sidewinders to it once they had realized the need for those.