Strength, or lack of, is generally not what kills the 5L40E - BMW even hung it behind up the M57 with over 40hp more in the X5.
I spent quite a bit of time researching this topic last year (didn't I post about that here? - I'll have to check). The problems result from a lack of adequate control pressure due to the valve body wearing prematurely. This apparently results from the aluminum alloy in the valve body being somewhat soft. As I was told by several experts, all the issues that are often identified as causes are simply results of this low pressure.
I'll try to find the writeup I did and repost it in this thread.
Edit: Doh. Found it. I posted it in a German RR forum (I can copy and paste from an English forum to a German and most will understand. Don't think that works in reverse...). In a bit of a hurry now - will try to find the time to re-write in English
RRC 2Dr, RRC 4Dr,
P38, and 2 L322s
(wife thinks I'm nuts - prob right, too)
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