I see three possible upgrades:
1) Alta has made some prototype, which they aren't selling yet, but
@Butch got his hands on one. I am not optimistic about it. It uses the Brembo clutch lever and is not plumbed into the rear master cylinder, so it eliminated the foot brake, until they figure out a new design.
2) Clake has fancier models, more expensive, with some special cams, which increase the leverage exactly where you need it. If the claim holds, this would be the most straightforward solution. But it is expensive to verify, some of Clake's prices are astronomical.
3)
@Mark911 has adapter a 2-piston rear caliper from the Superduke. It requires a custom bracket. If the foot brake is preserved, it will be too crabby and powerful. The foot lever ratio will need to be modified, then it will be perfect.
4) Two separate brake calipers? Ideal performance, but too heavy.