Have you ever tried fargesia denudata xian 1? This seems to be hardy enough for zone 6 and it also seems to be upsizing pretty well going from 2.5ft last year up to around 4-5ft this year. It's supposed to have a potential of 10-15ft and I think it might be the one to get. I do notice that this species tends to make fewer shoots than fargesia rufa, but it makes them larger.

I also think it is better to get larger divisions as they tend to be much hardier. Also stay away from tissue cultured bamboos as they are reputed to have inferior cold hardiness.