I've had my Bulli for a couple of weeks now. Today I took delivery of some new wire for it. I went for something a bit thicker (0.17mm nickel chrome) than what was supplied in the kit (0.15 mm).
The difference is quite dramatic. Firstly, because the wire is slightly thicker I found it easier to make up the coil. And secondly, when it's lit it produces a lot more vapour (you can also hear the difference when it's burning).
Don't know if 0.17mm is the ideal, but it certainly seems to be an improvement on the 0.15 that was supplied.