Alright, a few more tweaks and I'm pretty happy with standard HDD performance. Absolutely no stutter that I could see during gameplay (I saw a bit during playback of the video I recorded, but I don't know if I just didn't see it during gameplay or if it was caused by the recording since I'm recording to a 5400rpm storage drive.)
I've got the video uploading but my connection is being shit and saying it'll take 3 hours.
If anyone is interested in testing the tweaks I'm using:
BMSystemSettings.ini:
http://pastebin.com/4My2Gra6
BMEngine.ini:
http://pastebin.com/WyaHCHZE
Using Nvidia Adaptive Half-Refresh Vsync.
Interesting to note, that no matter what I tried I couldn't ever get the game to a locked 60fps, even after dropping the resolution to 1600x900. It would always dip into the 40s during super fast Batmobile runs, or fast gliding around the city on the HDD, and into the mid to low 50s on the SSD.
Oh, and this was downsampling from 2560x1440.