Given Guo Jing's martial arts achievement at the end of LOCH, how would he stand against the Five Greats if he were to partake in the first Mount Hua tournament?
During the second tournament in Mount Hua, GJ was already a strong match for HYS and HQG, being able to take on 300 stances from each of them. So if GJ were taken back in time to the same event 25 years ago, it is very likely that he could overcome the Four Greats, although he would probably still lose to WCY.
My assumption here uses HYS as the reference point and the fact that the Four Greats had always been around the same level. In ROCH, when HYS taught Yang Guo his Divine Flicking Finger, it was stated that HYS was almost 80 while YG was not yet 20. At that time, it was commented by HYS himself that he had achieved the equivalent of YG's current level only after he had turned 30. But YG could not even defeat Li Mo Chou then. Also, when the first Mount Hua tournament took place, HYS was only in his mid-30s. Unless his martial arts drastically improved during the several years in his early 30s, his ability at the time of the tournament might only be slightly better than LMC. And in no way could LMC exchange 300 stances with a Great in LOCH.