I'm intent on picking out an RD this weekend. It's interesting... I found the chain width spec's.
My chain varies between 7.3-7.5mm at the post. A 9 speed derailleur may be asking too much. Although, looking at a 0.040"/1.2mm, the span doesn't look like too much.
- 6 speed - 7.8mm (all brands)
- 7 speed - 7.3mm (all brands)
- 8 speed - 7.1mm (all brands)
- 9 speed - 6.6 to 6.8mm (all brands)