As I read this, a pilot doesn't get grounded for BMI > 40. BMI > 40 simply means his medical exam has to include checking for sleep apnea.
Since sleep apnea is already apparently a grounding condition, the only question is if BMI > 40 is a reasonable risk factor for sleep apnea.
Looks like it is. In this survey of data:
"Obesity and OSA
•About 70% of those with OSA are obese (Malhotra et al 2002)
•Prevalence of OSA in obese men and women is about 40% (Young et al 2002)
•Higher BMI associated with higher prevalence
–BMI>30: 26% with AHI>15, 60% with AHI>5
–BMI>40: 33% with AHI>15, 98% with AHI>5
(Valencia-flores 2000)"
http://www.stanford.edu/~davesv/Weight%20&%20OSA.ppt
The rate among the morbidly obese could be even higher. In this study of pts with BMI 35-50, 77% had sleep apnea.
Evidence supporting routine polysomnogr - PubMed Mobile
It looks like BMI > 40 might have been a fairly generous trigger, actually.