The 2" number is not a hard and fast rule. Determining factors include the roof pitch, ridge ventilation, size of roof, climate, etc.
The purpose of the air space is fourfold:
1. Preventing moisture damage
2. Enhancing the service life of temperature sensitive roofing materials
3. Preventing ice dams
4. Reducing the cooling load
Your roof construction schedule reads:
Outside air film
Shingles
Roofing felt or tar paper, etc.
Deck sheathing
Air space
Rigid insul.
Inside air film
However, you have not addressed the issue of:
Protecting the insulation on the interior side
Reducing the heat loss from the uninsulated framing members
Preventing moisture transfer from the interior into the roof assembly
Additionally, the use of rigid insulation between the framing members, rather than batts or loose fill, will be difficult to hand fit tightly enough to avoid heat and moisture transfer between the insulation and framing member (2x8) due to the gaps that will likely result.
I would suggest the use of an interior insulating sheathing beneath the framing members to reduce the heat transfer, as well as to provide a continuous uniform surface to provide a vapor barrier. Another approach would be the use of batts, loose fill or a spray-in insulation, rather than the rigid insulating board.
I would also recommend a gypsum board interior finish surface (1/2" = 0.45 R), perhaps in concert with a second air gap of 1/2" (R value varies from about 1.1 to 2.2).
For cold climates I would also caution you to carefully address the wall ceiling juncture. In this area the insulation is frequently compressed as well as having the roof ventilation compromised. As a result, mold and mildew frequently form at this location, starting in the corners.
In comparing the 6.5" insulation versus the 5.25" (is it readily available in these sizes?), the difference in heat loss (insulation only) is 520 btuh per 1,000 sq.ft. of area, based on a 70 degree delta. While I wouldn't characterize this as insignificant, I also wouldn't fixate on this single component of your structure, especially in view of your use requirements - whether it is full time or occasional. A single leaking window will lose as much or more than this.
You may also benefit from advice from a local Architect as to typical construction standards, particularly one who deals with energy efficient residential construction.
As always, offered only as opinion
