57x, I agree. Bikram said he likes that initial bite, so that zx6/14 caliper might be a good option . That's the other part. If you paid 150.00 for a set, your not trying hard enough.
It's a fun project for a bike in how there are so many decent parts we have access to. If you don't mind digging through eBay, you don't even have to spend much.
Another nice caliper, and the one I'd choose, is the caliper from the newer (2012-2017) zx 636 bike. But, not the 600 asmit has the older calipers. It's a Nissin monoblock. I believe it has 32mm pistons so it would match your master cylinder.
Thats the problem with what you have now. The Kawasaki monoblock use a 30mm piston. That's fine, but that needs a 17mm master cylinder. Your 19 needs to run a caliper with larger pistons.
Very light, and strong. All else being equal, I'd rather have a Nissin branded caliper than Tokiko. I don't know if performance is much different, but the quality level you see is much better, with Nissin.. The machining looks better finished, and the pistons appear to be higher quality.
2012 Ninja 1000