No. The cpu sets the fps limit. It's got to decode/process the game code first. It can only do that so many times a second. Does not matter what the resolution is, that's all on the gpu. The cpu will process the same code X amount of times per second for 1080p or 1440p or 4k. After that, it's all on the gpu to reproduce the picture and live upto the limit set by the cpu. Most times, the gpu doesn't have the ability at higher resolutions to reproduce the amount of pixels as fast as at lower resolutions. There's @1.7x as many pixels in 1440p as at 1080p. That's what drops fps when changing resolution higher, and the needs of a more powerful gpu.
Witcher 3 is not only bandwidth intensive, but also high thread count game. It's brutal on any cpu, but more especially on quad thread cpus as the already saturated bandwidth per core now has to prioritize data, which slows down IPC and therefore fps. A move to i7, 8 threads can alleviate the backlog, but not as well as moving to 8 cores, as even hyperthreading is just splitting bandwidth 2x per core to handle the 2nd thread.
You can't boost fps. That's already limited and capped by the ability of the cpu. 'Boosting fps' is a falicy as such, all you can do is increase the gpu ability to match fps the cpu can output. The fps is already there, you just need a gpu strong enough to get it.