Is the roof sheathing directly touching the roof insulation with this product?

In zone 5 it's possible to build Net Zero Energy houses at a whole-wall R of ~R30 or so. If that R38 includes the thermal bridging (or at least most of it), it's enough for most purposes.

An R44 roof is better than code if that's the "whole assembly" number, with all thermal bridging accounted for.

In zone 5 a 1" vent cavity on the exterior side of the outer skin of a roof panel between the outer skin and a nailer deck would be sufficiently protective, assuming it's skinned with OSB on both sides.

Your window & door options sizes will make a bigger difference in heating/cooling than a vaulted ceiling or 8' vs. 9.5' ceilings. Every square foot of U0.28 window loses as much heat as 11 square feet of R38 wall (whole-wall-R).

.5-.7ACH? You didn't mention any window choice, that number dosn't sound very usefull without more detail about the total wall makup. Of course workmanship has a HUGE effect on this end number.

Vaulted ceilings can make it a little more difficult to get higher insulation values. Does this roof plan extend the ceiling insulation out over the wall to overlap with the wall insulation(no low R value at the top of wall/edge of ceiling)?

As for heating comfort, larger room volumes typically need more air movement if heating with forced air. IMO radiant is more effective in larger room volumes. Instead of pumping in warm air that just passes up to the upper room levels, raidant floor or walls makes the room feel warmer without heating the air as quickly. Radiant floor when it heats the air is applying the heat to the typically coldest/lowest air in the room.

As mentioned windows will have a big effect on the overall R value of the envelope. U0.28 = R3.6 U0.19= R5.2. Their design and quality will have a large effect on air infiltration.

prefabricated panels(compacted cellulose) claims .5 to .7 ACH @50Pa for the finished product.

That's a claim I would get in writing.