I'm not sure mine are even real Brembos. I have one set of 330mm rotors from the new zx10. The other set is from the 2016 zx14se. Both are round...no idea if they are the real thing, or something made to a Kawasaki price point.
With that said, the sizing comes to mind. If you do the math on it, you gain about 3% more power for every 10mm in disc diameter you add. You not changing the diameter does make sense. There's not a lot to be gained from doing so.
Then again, it's basically free power. The spacers look good, and don't hurt anything. With a 310mm disc, it's a 5mm spacer and you use the stock bolts. I have 15mm spacers, on my ninja, and it's no big deal. You and I have this love for high end parts, I know....
I think going from a stock Kawasaki rotor, to a Brembo (same size) is not really worth it, unless your stock rotors are damaged. I find my fake Brembos to be very smooth, but I really don't know if I could find them if we were having a blind sample with these vs stock. Mine appear to be very similar to the oro, and I really like them.
Seriously, if I had to change something first, I think it would be calipers. I think the high end Brembo rotors may have a racetrack advantage. Maybe they handle repeated stops better than our stock rotors, but not so much on a one-for-one stop.
Your master cylinder is a perfect match for the Brembo m4. It's too large for the gp4rs. I think I would buy the m4's, first, and consider a rotor uphrade when you need to replace due to wear.
If you have to, I think both the rotors you discussed will be excellent.