Alright, sorry for the delay. It's been a busy weekend.
I just have a few questions, are you talking about GTA V Online?
You shouldn't be getting 40-50 FPS on Ultra in this game at 1080p. Anandtech got an average FPS of 55, minimum of 38.5 with what sounds like similar settings at 1440p. (I can't seem to use BBcode to have in-text images, so here are the links to the GTA V Benchmarks I looked at. With a quote explaining their graphics settings.) You might also want to take a look at the full review from each, to further compare results.
Anandtech GTA V - 1440p, 4xMSAA, Very High
Full Review
On a quick note about settings, as Grand Theft Auto V doesn't have pre-defined settings tiers, I want to quickly note what settings we're using. For "Very High" quality we have all of the primary graphics settings turned up to their highest setting, with the exception of grass, which is at its own very high setting. Meanwhile 4x MSAA is enabled for direct views and reflections. This setting also involves turning on some of the advanced redering features - the game's long shadows, high resolution shadows, and high definition flight streaming - but it not increasing the view distance any further.
Guru3D also had a benchmark which suggested you should be getting more FPS from your GPU, though admittedly they used slightly lower settings.
Now, I'm fairly certain both websites used the built-in GTA V benchmark, so if you could personally run that benchmark and post results from that, mimicking the previous reviews' settings, it'd be interested to hear.
Something to keep in mind, both reviews used a more powerful CPU, but regardless, the fps difference shouldn't be this great.
I think we should also start by having you run a GPU benchmark, to help isolate the source of your performance issues.
Try installing the Unigine Valley benchmark:
http://www.guru3d.com/files-details/unigine-valley-benchmark-download.html (download mirror at bottom of page) and let us know what your score is. This should directly measure GPU performance, while minimizing the impact of the CPU. You should score around 5900 on the Ultra Preset at 1080p with Antialiasing disabled.