jerry, as Dutchman said you are measuring in the right place. I find that surface temps are around 100% to 150% less than internal temps. My magnum runs at around 300*F at max burn, so your 400* is high IMO.
Jeremy has the right idea with increasing to 8", that relieves the static pressure and allows more flow. Nice job Jeremy.
With regards to a larger fan on the Harman, I feel that will not work since the interior design of the plenum is for that fan, just enlarging the opening will do very little for improving flow. A better bet would be to try and eliminate the static pressure being built in the duct due to length of pipe, fittings, and vertical rise. Remember air is a fluid and needs a large enough pump, since the induced loss is exterior to the stove, the solution would be best exterior to the stove. after verifying lower temps by dumping the heat out the front you can try a metal bladed 8" fan at the top of the run, if you adapt the 8" fan the corresponding drop in static pressure at the fan should allow it to operate efficiently enough to reduce the duct temp to safe operating limits. Just remember that static pressure is the enemy to this type of set up, you want to open it up as much as possible and allow the fan to work.
And yes, IMO you would be able to increase the output dramatically if you get the air flow up and add a coil. Your current set up is basically causing all the btus to go to the path of least resistance instead of being put to work. The ironic side effect is that the grates are stying much hotter than they should.
Keep us posted as you solve this intriguing mystery.