If you are restricting your options to Nvidia, I think upgrading (especially if using EVGA's step-up program) to a 780 Classified is the best option. 1440p will benefit from that extra GB of VRAM over the 670/680 2GB SLI setup.
As for low minimum FPS, I think overclocking your CPU will help with that. You should try it if you haven't already.
As for 780 FPS numbers, the reviews don't really give the most accurate measurement of the card's performance imo. Different 780s boost to different values when run at stock so the review numbers may not be very consistent.
Also, as far as overclocking goes, reference GK110 cards are limited by their low power limits and easy throttling at those limits. With a modded BIOS, non-reference 780s will do a much better job than a reference card with reference BIOS.
As for Grid 2 performance, I'm currently playing it with my Titan at 1150MHz on a 1080p screen. Graphics settings are maxed out with 8x MSAA (8x, just because I can). FPS stays locked at 60FPS at all times and most of the time the GPU downclocks to below 1GHz core clock since the game doesn't require that much power.
780s require about 150MHz higher than a Titan's core clock to match a Titan's performance. Classified 780s are hitting 1300MHz left and right (check the Valley thread) although there are exceptions. At 1440p, you won't require anything close to 8x MSAA so even with ~80% higher pixels than 1080p you'll be maxing the game out with ease. So you'll be getting the same performance that I'm getting.
As for my experience with games with the highest of requirements, I've played Metro Last Light at 1080p with my Titan at 1150MHz at maxed out graphics settings except for SSAA which was set to 2x. At these settings I saw FPS to be at 50-70 most of the time. You'll get the same performance FPS-wise if you drop the SSAA. [Note : 2x SSAA means the game is running at twice the native resolution of the screen. So basically I was running at 2x1080p resolution which is higher than 1440p. So 1x (or 0x, I don't know what its called) SSAA that is to say no SSAA will yield 50-70 FPS at 1440p.]
I personally don't prefer/ wouldn't recommend a dual-GPU setup (more heat, more power consumption, not all games scale well - to state a few reasons). But if you have no problem with that, I think a 7990 (as suggested by others) is your best option right now. The 384-bit memory bus with the 3GB of VRAM is better than a 680s 256-bit 2GB VRAM. Also, CF is known to scale better than SLI afaik (don't quote me on this one though, as I am not up to date with recent scaling numbers).
Hope this helps!