In US building material terms K-value ="1/R value per inch of thickness".  (I've been mis-stating it- mea-culpa! The K-value of the high-perm foam is higher, not lower than the K-value of most other closed cell foams. Its the R-value per inch that's lower. The typing fingers are quicker than the mental-editing... )

Venting the roof deck from below will only keep the roof deck marginally drier than with a foam/fiber approach,  but it limits the total R value to something that performs R33-R35-ish on it's best day, allows critter-access to the fiberglass, allows more convective loss of R-value when at the temperature extremes, leaks air from the exterior requiring more detailing to create a (more susceptible to damage) interior air-barrier, etc.  

Batts degrade over time. I wish there was an easy way to rip out & replace the HD batts installed in a vented cathedralized ceiling in my house without tearing it apart.  In less than 12 years it's had both mouse and bat infestations despite hard-screened soffit & ridge venting, and it's easy to spot the developing heat leaks from the compromized  batting in the snow-melt patterns. This has not occurred on the un-vented foam insulated cathedralized ceiling sections.  When I re-roof I'll blow cellulose into the vent gap from the exterior compressing the batts and add sufficient exterior rigid foam to mitigate the seasonal moisture issues, but until then it's not so great.  (YMMV, but don't count on it.)

Closed cell foam at 3 " thickness adds structural strength to the assembly, creates a near-perfect & well protected air barrier at the exterior (that will be air-tight in a century), becoming the exterior air barrier necessary for the fiber perform to spec.  It also allows you to use cellulose for the fiber layer, which protects the assembly further by buffering seasonal  moisture and reduces convection within the fiber layer by an order of magnitude compared to high-density batts (almost 2-orders of magnitude compared to low density batts),  has no gaps compressions or voids with a perfect fit, never attainable with batts, and adds as much thermal mass as increasing the thickness of the sheet rock by 1/4-3/8".

And, ~R50 is a 40% performance boost over ~R35, and it's a performance level that will still be there in 50 years.  (In whole-assembly terms with the thermal bridging of the rafters factored in it's probably only a 30% boost, but that's still significant.)  The big uptick in expense is the foam, the wet-sprayed cellulose blown-in-blanket has a similar installed cost per unit-R to high-density batts. 

Between the air sealing to the exterior and extra rigidity of the closed cell foam, added to the mass and vibration absorption of the cellulose sound transmission through the assembly is also much reduced (which is a big plus under a metal roof in a hail storm, or even a rainstorm- but I know you never get those in Seattle. )

In wall assemblies wet-spray cellulose is usually cheaper than high density batts since at lower densities it's sprayed directly without neeting to install mesh as a prior step.  If you're detailing the exterior sheathing as an air barrier, there's little value-added to dense-packing the cellulose in walls with 30-50% more material, which is sometimes done for limiting air infiltration instead of building a true exterior air-barrier.  It'll have a slightly lower center-cavity R than high density batts, but the perfect fit of blown/sprayed fiber mostly makes up for that in real-world performance.

Thanks Dana1.  Great advice.  So, the entire roof stack up  would be  (from the outside in)

1)  Standing seam steel roof

2)  Interwrap Titanium PSU-30 roof ice barrier

3) 1/2" CDX plywood roof deck

4)  3" of Icynene MD-R-200 higher perm closed cell foam sprayed directly onto the lower surface of the CDX roof deck

5)  8" of wet spray cellulose to fill the cavity

6)  sheet rock

Did I get that right?

Question:  Since, I'm trying to dry to the interior does the sheet rock type and paint matter?

Also, Is there a problem with doing in-ceiling canned lighting with the cellulose and foam insulation?

Thanks for all the great advice!!

Sporto

You've got the stackup right.

Sheet rock is highly permeable to water vapor, and presents no drying problems on it's own.

Paints and interior finishes will differ though. Alkyd/oil paints are often highly vapor retardent and should be avoided, as should vinyl or foil wallpapers, etc. Latex & latex-acrylic paints are in the 2-10 perm range, somewhat vapor retardent, but still plenty breathable, and not a problem.

WA code requires air-tight insulation-contact rated fixtures for can-lights. If you're putting 300W bulbs in them it's still possible to cause problems, but with high-efficiency LED or CFL bulbs, not so much. Pin-base CFL fixtures with dimmable ballasts provide the highest efficiency/lowest heat, but if you like sharp-edged shadows LED downlighting would be better. For high quality ambient light, dimmable T5 or T8 fluorescent indirect uplighting from coves/valances utilizing the walls an ceilings as the light diffusers gives high efficacy- better visual acuity at any given light level, and can allow you to use lower power. The color temp and color rendering needs to be paid heed when using any of the high-efficiency lighting, but the good stuff DOES exist, and is worth paying for. Can light downlighting often introduces glare factors, lowering overall efficacy, and they make for thin-spots in the insulation. Use them only sparingly/advisedly.

Thanks!