The Firestrike score is close to scores on working systems so your performance isn't related to a hardware issue.
You can easily validate that with other games. Running a MEMORY test is probably pointless though I always recommend doing MEMTEST at least once to validate the settings and there are no errors.
This suggests it is the game that is the problem.
http://www.gamespot.com/articles/gta-5-pc-frame-rate-problems-resolved/1100-6429043/
"Eight days since it was first reported that Grand Theft Auto 5 on PC had suffered from "considerable" frame rate issues due to a recent update, developer Rockstar has deployed a patch that it claims has fixed the issue."
*So supposedly a patch was just released within the last couple days which should be later than the problematic v1.28 patch. I don't know if Steam or other methods of distribution have this available yet as I don't own the game.