Just because you're using a prebuilt doesn't mean you can't upgrade any of the components (unless it's a laptop, that is). You would be able to upgrade basically any component on that rig if you wanted to so long as it fits.
As to your issues, check the temps on your CPU and your GPU when you start to notice the stuttering. From what I recall, the ROG GR8 models are smaller PCs so this may be an issue with temps spiking somewhere more than anything, which would cause thermal throttling to kick in. Generally speaking, stock coolers (which most prebuilts use) are pretty bad at keeping cool for extended periods of time at high load, and this is especially true with smaller PCs. You can use things like HWInfo to check the temps of your components, just keep it running and then minimize your games when you start to notice a lot of stuttering to check the temps of your GPU and CPU. For GPUs, this generally doesn't kick in until around 90+C depending on how old it is, and with CPUs it's usually around upper 90sC. If your temps are spiking that high, then it's probably time to check your cooling on your PC.
Another issue could also be HDD speeds, since Just Cause 3 and FFXV are generally huge open world games they might have issues trying to quickly load high quality textures and files which could result in stuttering. You could try lowering the graphical settings, and then see if the stuttering continues to happen.
