depends on the game if he plays sc2 then its a different story.
well ofc game optimized (with the feets) for single thread are legions, hum i play also a lot of RTS but SC2 never got my attention xD well yes in a MMO in a crowded area the fps drops, but even in a 40 people raid the fps is still enough, Firefall for example, in a town i drop to 35-39fps but it's still enough Rift/Tera/FFXIVARR same but once out of town even with a decent amount of player, it's back to 60-110 (depending the game, Tera is a prime exemple of AMD hater xD,still runs fine tho)
8300 series fixed alot of what was wrong with the 8100 series. However the 8100 series is still a current cpu. A true upgrade from that platform would be skt 2011-3
for somebody with a 9590 (day 1 buying) and a ASRock 990FX Ex9 (or any "expensive" mobo)indeed a 2011-3 would be a upgrade (affordable in that case ...)
back on the topic, the OP has a good mobo and a not so bad CPU so why recommend him to change mobo+CPU, because it is AMD? because it's not a "gamer CPU"? both argument are debatable... yet a eidairaman1 said it's still a current CPU and can hold on it's own on 3 CPU generation from Intel (2xxx/3xxx/4xxx) yes i mean it, since it proven, for example my friend has a i5-4670K and a 770 yesterday i was meeting him for a small "LOL LAN" we compared his rig with my 6300+ R9 290 in many games well ... not much "real" and "noticeable" difference (debatable point again but "feelings" are not numbers on a benchmark ).