I measured the controller current by making one measurement with 2 Hash boards connected and then with just 1 hash board connected. The difference is the current that a single hash board is taking. Multiply by 2 and take it from the number you first thought of
& the have the Controller current.
So the controller takes just over 0.4A, so controller Power is only 5W. So unless there is a flaw in my methodology not much of a power saving from sharing the controller.
I think your methodology sounds correct, so I've stopped being lazy and just run a couple quick tests to compare. I'm using a Watts Up Pro ES to monitor power at the wall, and I have the fan hooked up to a separate power source - so the only things that are being powered are the BB, controller, and two blades. All settings are the factory default, PSU is the Corsair HX850i at 110v:
Bassline - 558w: I booted it up and had it mine for a couple minutes like normal, hashing is right within spec. Keep in mind that the fan is NOT included in this.
One Blade - 275w: Physically unplugged both the controller connector and PCIe power plugs from one blade - software ran like normal, although the hash rate was obviously 1/2 of normal.
No Blades P1 - 14.4w: I left the PCIe power connected to only one blade, but unplugged the controller connect - so no blades are actually connected. Everything booted up fine, but no hashing.
No Blades P2 - 17.6w: Similar to No Blades P1, but this time I hooked up PCIe power to both blades, but left the controller connectors unplugged on both. Everything booted up same as with P1.
Zero RPM P1 - 68.3w: This is running with their latest firmware, one blade connected correctly, but without the fan connected so it won't do any hashing.
Zero RPM P2 - 127.1w: Same as Zero RPM P1 but both blades fully connected correctly, but without the fan connected so it won't do any hashing.
Bassline+Fan - 580w: Latest firmware, fan set in software to 100%, both blades connected and hashing correctly.
.... So, now some insights from the numbers - a couple things stand out:
#1, with a platinum PSU (at 110v no less), it's beating the spec and doing 0.502W/GH (at least this particular unit)
#2, the overhead of a blade is 3.2w, regardless of it doing anything
#3, the BB+controller is somewhere between 8w and 11.2w
#4, the overhead of a fan (full speed) is 22w
#5, with the new firmware, when it benches a blade, it still consumes 58.8w
And back to the original comments about system savings, being generous the S5+ saves 89.2w (6 CPU/controllers and one fan) over 7 S5's.
Anyway, hopefully other people find these numbers helpful, or at least interesting...