Really, I've had numerous problems with 7 speed RD's not shifting onto the big cog in the back of a 9 speed system. Usually with lower spec'd stuff like Deore it'll reach, but XT's/XTR's have always ended up with problematic shifting.
I'm not so proud to say I'm absolutely right. Might also be due to the jockey pulleys.
I will add that I have NOT tried a 7-sd on an 8 speed system conversion in probably about 4 years. Just when I worked in shops off and on over the years we shyed away from this due to problems when 9 speed stuff first came out and conversions were more common.
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