PCIe scaling isn’t really significant. With a 4090 you lose 2% going to 3.0 instead of 4.0 at 1440p per techpowerup https://www.techpowerup.com/review/nvidia-geforce-rtx-4090-pci-express-scaling/28.html

He is using and instead of nvidia and linux instead of windows but his card is so much slower that the drop should be a lot less

From some other benches people have posted, ultra settings at 1440p he should be averaging 180 fps then even if he is using 3.0 x4 due to lane splitting from ssds which is roughly equivalent to pcie 1.1 x16 then it’s probably still not the main culprit

Phantom Spirit from Thermalright performs a bit better and costs roughly the same as the peerless assassin

Hidden Power doesn’t exist in Gen 9. Although you can use Tera Blast with the appropriate Tera type to replicate that. Note that using your Tera on Heatran will also change your defensive typing for the 3 turns (?) it lasts.