I only checked my MPG once, when I first got the bike. I think it came out to the low 50's like you're talking about; it was definitely between 50 and 55 mph I know that. That was under moderate riding, under 6K RPMs.
I've ridden my down to the flashing symbol on the gas gauge a few times. One of the times, I must have gone a little too far, and I almost had the engine die on my while riding it. I've been more careful since then. But I'm surprised to hear you had some symptoms of that with one bar left.
Is the 750 a lot more powerful than the KLR then? I don't know anything about the KLR. A lot of the other riders I run into think the Z's 750cc engine means it's in the same category as the new supersport 750s, like the GSXR, but I think it's probably a bit tamer than that. I'm curious to ride a supersport now just to compare them, don't know if i'll get a chance.