Upgrade 870=224???

[get2bill]

Long story short - Trying to address gaming issues for Grandson's 870-224.  Slow, play, graphics issues, "glitchy".  He's become a very fast fortnite player, insane builds, edits, etc.  Replaced the HD w/a 1 TB SSD and doubled the memory to 16Gb.  Huge improvement in graphics and overall game play but still kinda "glitchy" as he calls it.  . He also plays at our house a  lot as well on a newer PC ( a 875 0014) and it plays "smoother" aka responds faster (done side by side tests at our house).  The GPU is actually the same in both so I'm considering swapping out the CPU (his is a I5-7400 (4 core, 3.0/3.5 GHz,) and ours is I7-8700 (6 core, 3.2/4.6 GHz)

Here's the issue:  The best CPU that his mobo will accept w/100% compatibility is I7-7700.  The upgrade from his I5-7400 will provide the following improvements:  3.0/3.5 MHz > 3.6/4.2 MHz, adding hyper-threading so 4 > 8 threads, and more L3 cache.  So my first question is: So now that I've already forked out cash for memory and SSD, do you think this $300 upgrade would make a truly noticeable improvement?   Comparing the $300 I7-7700 to the I7-8700 in our pc there is still would be a pretty good delta (2 more cores, 4 more threads, lower base freq 3.6 in 7700 and 3.2 in 8700 but higher turbo 4.2 in 7700 and 4.6 in 8700).  So I cannot exactly match what he's playing at my house where he is 100% happy with it's game-play but can make the aforementioned CPU improvements.  Don't have the bucks for a new high end machine,I've already added memory and an SSD but dropping an add'l $300 in this machine w/no guarantee of nailing it worries me (plus that's a lot to put in a 870-224.  What would you do?  Appreciate any and all thoughts...  Thanks!

[hwsense]

Your best option is i7 7700, you will not be able to go to 8700 as its a different generation and the motherboard will not be able to support it. The I7 7700 is a pretty capable CPU still and higher clock speeds will definitely help in games, I had one of them in one of my PCs till recently and it was very good for the majority of the games, he will certainly not have issues in Fortnite. 

No as the majority of the games rely on GPU power instead of CPU power these days, what is the GPU that he has and you can probably upgrade this as well, I assume is the GTX1060, which for Fortnite should be absolutely fine too.