IME, roaming connectivity speeds have nothing to do with the home carrier, but I'm not an expert on this.
Switching topics, during my next trip to HK (in ~10 days), I want to stock up on prepaid data sims that work in China. I am interested in recs regarding specific channels in which to obtain these at the lowest price point. I would prefer vendors near Wanchai, but I don't mind going to Stanley...hoping no Mongkok though.