I recently upgraded my Gtx780 to a 1080ti and also bought a 40 inch 4K monitor. Suffice to say 2K17 looks incredible. Ironically I couldn't run it at 60fps & 1080p on my 780 but now with 1080ti I've got a rock solid 60fps @4K.
It's definitely true that 2K17 ran a hell of a lot worse than 2K16, I suspect it was far more hungry for VRAM. I only had 3gb on the 780 and so would constantly get stuttering and dips into the 50s. The 1080ti has 11gb and now the game never feels like it's breaking a sweat. All with the same CPU I've had since 2013. A 4670K overclocked to 4.2
I don't think you're looking to burn money but the outlay needed is pretty steep. Also consider that even with the 1080ti we still don't have a single GPU that can run every game at ultra settings and maintain a locked 60fps. Even really well optimised ports like GTA V need tweaking to get there. If you're happy at 1440p then maybe it would be wise to wait another year or two? My reason for upgrading was simple. I hadn't touched my system since 2013 and I scored a big tax return in addition to finally getting a pay rise. I realise I'm in a very lucky position to be able to do so.
That said, I'm not thrilled with my monitor choice. I went with a 40 inch Philips monitor and I'm already frustrated with the amount of ghosting present in dark scenes. Not a problem in 2K but in a ton of other titles it is.
I guess the perfect rig is always just around the corner...