It's really curious how different brands of levers have different pull lengths (and hence probably different mechanical advantages) but aren't advertized, I've always thought it would be interesting. Anyway, since they are Shimano STI, my 2 pennies worth is:
i. obviously keep rims clean and pads clean with good condition cables (sorry I know it's obvious but have to put it in)
ii. change to a better quality brake pad (e.g. I'd recommend Salmon Koolstops or similar -they are a softer compound that give really good braking performance) for less pull effort
iii. twentysix by twentyfive mentioned it above, but cross/inline brake levers could be a good idea as well. These are brake levers that are mounted on flat part of your bars inline with the brake cable that allow for more "traditional" hand position braking (see
View: https://www.youtube.com/watch?v=K2ZgNrS7hSc
). I have these on my touring bikes and find them pretty good.