Xeon Hex-core x5660 OC'd to ~4.3Ghz
24GB 2055Mhz DDR3
1TB Samsung Evo 850
& a EVGA GTX 1080 SC with a +90 Core & +325 Memory overclock over & above the normal overclock this card comes with. I use dx11. Modded photovision enb/sweetfx mostly delivering a bit of HDR. My timecycle is highly optimised right enough ('fragment.xml' & 'procedural.meta' optimisations also. See 'Optional:' below), although with '<far_clip>' & '<water_reflection_far_clip>' set at 20000 in all weathers.
I've got Beta Vegetation & Props & GTAV Remastered mods (both optimised specifically for my game though, so not tanking frames in certain places). Looks awesome compared to vanilla but still runs with higher frames (R*, what were you thinking?!), so not sure how valid this is comparing it to your game.
I game at 4K but setting my resolution to 2560x1440 & maxing everything (no frame scaling) I get pretty steady 60+ frames (I didn't switch off Gsync/Riva Tuner to test max) in Rain weather (most intensive weather I have) with dips down into the low 50's in certain spots (2 or 3 places around the map that my machine has never liked. Remove/thin out trees to fix is a safe bet).
To me your frames do look a little low. I would expect you to be hitting 60fps most of the time with your specs (although a little overclock to a GTX 1080 can do wonders for GTA V's minimum frames ) & that's quite a jump (+65>+95fps) going to dx10! You're really just losing AA & very high post processing (dof etc) dropping to dx10, not much else. I would get +20>+30fps doing that not +65>+95fps so it might be worth looking into why dx11 seems to be eating resources like cookie monster.
As mentioned, I don't know how diagnostically valid all my info will be, mine is a very unique game , but I would definitely play around with your settings on dx11 & see if you can figure out what is possibly eating those frames?
Before continuing, here are a few things to rule out:
Download & use Display Driver Uninstaller (in Windows Safe Mode) to uninstall your GPU driver & then redownload (don't use the same copy if you have one stored) & install the newest driver.
Make sure 'GTA5.exe' is set to 'High' priority in Task Manager (Task Manager > Right-click 'GTA5.exe' > 'Set CPU Priority' > 'High'). You can use a program like Prio to set the priority permanently so that 'GTA5.exe' starts with 'High' priority every time (it reverts to 'Normal' priority each start otherwise).
Run 'sfc /scannow' from elevated (admin privileges) command prompt. Instructions here.
Run check disk on OS & game hard drive (if different). Instructions here.
Have a look at this thread here also & compare your frames loss/gain to what they have & see if anything pops out at you. They won't have the same specs as you but you'll get a general feel for how each setting affects frames. If one of your settings is off by a lot that might be your culprit
Here's my Nvidia Control Panel Settings (optimised):
Ignore the GSync & Fast Sync settings. Definitely turn on MFAA if you are using Anti Aliasing (that tip applies to all games)
If you can't figure out what is going on, here are a few ideas on how you might increase frames without lowering the visual detail of your game too much (if at all).
Optional ('settings.xml' optimisations):
<Shader_SSA value="false" />
Add the lines into your 'settings.xml' here '...\Documents\Rockstar Games\GTA V\settings.xml'
Shader SSA is a bit of a resource hog without much benefit. Set it as 'false' to gain a few frames. Test the others as results may vary. They don't lower the detail of the game so it should look the same but hopefully run a little better in certain situations.
Optional ('procedural.meta' optimisation):
Note: This removes rubbish on the ground, small plants, seaweed, starfishes. Lots of tiny little things that eat resource. I gained about +14fps doing this. You may want to keep those things but if not open the file & make it look like this:
<?xml version="1.0" encoding="UTF-8"?>
Optional ('fragment.xml' optimisations):
Open the file & change:
Basically, reduces the draw distance for a few fences & some street signs. You won't notice any difference in-game as '500.000' is still pretty far away. Might get you a few frames.