At the end of the day - if the guy wants to be sure that his kit is covered by warranty and product liablility - then the only sure way, is to follow manufacturer's recommendations.
... and buy from an authorized dealer and have the components fitted by a Shimano certified mechanic?
I understand why Shimano publishes the specs it does, and I understand why they're conservative. If product manager A orders 1000 of derailleur B he wants to know they're going to bolt right on and work with cassette C - no fussing with chain length or b-tension. Shimano have to guarantee that. That's not REKI's case - he wants to put together a special system for a special ride, and wants to know what he might get away with.
"Someone on an internet forum told me it might work" is unfortunately not going to cut any ice with the warranty department, and any injury claims compensation if it all goes pear shaped.
That's true. But anyone who wants to know the official, straight-down-the-line Shimano spec can go to the Shimano website and read it off a chart. Anyone asking questions on an internet forum gets internet forum answers - things that work that aren't supposed to, a mix of bad advice and good, experience and hearsay.
If we'd all followed Shimano spec, we'd never have found the hidden click in our XT thumbshifters or put Ultegra freehub bodies on our XT hubs. Surely that's all part of what a forum like this is about.
Anyway, it's late. Let's join Agency Scum for an Ovaltine.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.