Cathedral ceiling 8/12 with 16 inch space between top and bottom  rafters.

This is the insulation thought with the draftsman:

R40 batts between rafters. 2 inch XPS foam board on house side of bottom rafter with osb board screwed in plave on rafter. Drywal or pine ceiling instaled on OSB. Due to OSB drywall could be 1/2 instead of 5/8.

Structure has full soffit opening at bottom and ridge vent at top.

Does this sound like a good system. It gives R 50 and eliminates all thermocouples.

I have searched on this site and google and found nothing like this.

Where are you building & what climate zone?
Are you venting the roof? If so, how? If not, why not?
Sounds like you have two rafters; if there is a space between them the batts will not fill it, so you'll be much better off with a blown in product - either cellulose or fiberglass. (Of course IMO fiberglass is an old and bad technology for many reasons; its only used because it is cheaper initially, and ultimately you get what you pay for.)
if you use t&g under the osb, you should seal all the joints in the osb; t&g is notorious for leaking air.