I'm unclear about exactly what happens with the motor, though I believe there will be some efficiency gain at the lower speeds with the slow wind motor. Partial throttle on a steep hill is dangerous for a controller due to phase current multiplication during current limiting, and the 4115 FETs in the high voltage controller will be less able to handle that heat than your other controller. The greater BEMF of the higher turn count 5306 may make it an easy enough load for the controller that your solution will meet your needs, but I think going from a 5304 to a 5306 and increasing voltage by 50% puts you back to the same motor performance curves. Lowering the voltage will move the peak performance in terms of power and efficiency to lower speeds, which I think is what you're after.
Once I tuned a high performance controller to handle my usual hills I bought myself some leeway, and I have enough speed to handle getting on the highway and enough torque to charge up 10% hills. Setting the current limits higher gets the motor and controller a bit too hot for my tastes, and setting the current limits lower prevent me from getting to the efficient speed on the same hills and again the both get hot. I'm running high enough power though that partial throttle up a steep still puts my controller at risk,
My road needs are going to require a second motor or a gearbox if I really want to go on mountain rides worry free. I'll probably accept the weight penalty and just go with another high power hubbie on the front, and then I'll own hills with a 15% or greater grade. An option that might work for you is a geared hub as a second motor to help with climbs, since it freewheels the rest of the time. They can't go to high voltage though, and for my main bike it won't work because I run at 74V nominal and will soon go to 88V nominal.
You should talk to ZapPat. He runs dual 9C's as his off road rig and loves it, and he has enough regen braking torque that he doesn't even use mechanical brakes. That would give you about the same weight as an X5, better weight distribution, higher performance, and double the surface area for better cooling of both the motors and controllers. Maybe there's been enough improvement in geared hubs for a dual geared hub bike to be a reliable solution for you, but I don't have direct experience to know. I like my silence, have a lot of DD hub motors, and don't mind the weight, so I'm sticking with direct drives for now. Plus I love the strong pull of a high speed wind motor that's fed plenty of amps all the way up to typical cruising speed, and above that I still have good acceleration and speed headroom for passing.
Regarding a hubbie shell as your battery box, I've got a scooter motor that is cavernous inside. The problem would be all that weight, especially in the steel backing ring for the magnets. A better option might be to find a source for motor covers for one of those newer large diameter hubbies like a Magic Pie. Then fabricate a much wider spoke flange to create a large internal space for batteries. I've always worried about vibration, so I'd rather build a frame specifically to enclose batteries, which would also avoid the unsprung weight issue.
John