I have a Z750 as well.
I find the brakes to feel very spongy to me. I dont remember noticing it when i test rode the bike or for the first few months of owning it - im figuring it is because i was still inexperienced as i hadnt ridden bikes for 5 years plus i hadnt ridden many other bikes.
Now that i have ridden a few other bikes i want my brakes to be better.
I have just ordered a set of braided brake lines from aus streetfighter
http://www.austreetfighter.com.au/
they are about $160 and i reckon thats a pretty cheap mod.
They havent arrived yet so i cant say how easy/hard they are to fit yet but as soon as they are on i will post about how it goes
i emailed the guys there about it first and they were incredibly helpful.
I would give them a call and have a yarn to them about what you want to do