I am planning on building a small house that will have a cathedral ceiling.  General details are that it will be approximately 14′ wide, and around 25′ long.  I am planning on having the roof with a single slope, like a shed roof.  I am trying to achieve around a R-48 for the roof.  Climate zone 6B.  Initially, I was considering 14″ tall I-beam style joists, 24″ OC.  I was planning on filling these with fiberglass bat insulation (R38).  I would plan on having an insulation baffle, using the shoulder of the I-Joist to create a 1-1/4″ ventilation space below the roof decking.  Below the insulation would be 2″ foam panels (R10), 1×3 spacers, the 1/2″ drywall.
After costing out the insulation, I started to consider sprayed in cellulose insulation.  I think if I upsize the rafters to 16″ thick, I will have room for 14″ of cellulose insulation, which gets the R-value up to R49 (assuming R3.5/inch).  This then eliminates the need for the 2″ foam board insulation, and the sleepers for attaching the drywall, as now it can be attached directly to the I-joist rafters.  This option should be less costly for materials.  I assume by eliminating the foam board, there will be some thermal bridging.  I am not sure how detrimental that will be to the overall R-factor.
Has anyone else built a ceiling/roof this way?  Any recommendations, or gotchas to be worried about?