Check out the radiant barrier 'double-bubble' insulation used in commerical construction. When applied between the rafters with a small air space above, it's rated R7 or so. Then apply your sealed foam board. You should be able to get an R20 even with only 3.5" of space.

I recently remodeled the second floor of my cape in MA. I did the 3 rooms 1 at a time. I only stumbled upon information on thermal bridging and had very little knowledge at that time but learned as I went. I sistered 2x10 to the 2x6 for strength and to create enough room for fiberglass and baffles. I then added a layer of Polyiso or XPS applied to ceiling joists or rafters to reduce thermal bridging and have been very happy with the results after 2 years. The Bathroom was just completed and due to head room concerns I did not sister 2x10 but used a modified version of a system featured last year in FineHomeBuilding.com.
First make air baffles out of 1" XPS. that are slighty narrower than rafter bay width. secure them loosely in the bays and use great stuff can foam to seal them completely.  The baffles provided a 3/4" air space above and added r-5 to the system and are not a vapor barrier. next I added 3 1/2" R-15 insulation which filled the 2x6 bay. Next a 3/4" layer of Polyiso (R-4.5 ?? i forget) taped joints of course which does act as a vapor barrier with the foil face. As an added bonus on the flat and level portion of the ceiling I added strapping to make drywall easier and this allegedly addes an r-1.5 to the system due to the air gap and reflective surface of the Poly Iso. I was much happier with this system.
I wish I discovered this forum before doing any of this work. I would definately use sprayfoam applied direcly to the ceiling joists in the future.