Two problems:
1. The IRC does not allow composition shingles to be overlaid over a shake or shingle roof ( I recall fire concerns). Cant see how OSB decking, then shingles would make much of a difference.
2. Will be difficult to insure (I know they wont offer windstorm insurance in Texas with shingles over shakes). Its gonna be hard to convince an inspector that the new decking is there as it will be hidden from view by the cedar shingles from below and the composition shingles above.

Just my two cents.