Application would be a large room where I want killer 2 and 7 channel sound, without towers in the middle of the room.

So 3 x SS50 up front, 4 SS3s for surrounds all wall mounted.

speaker level from amp to 2 FF3s on the front wall for left and right. set range to 50-100 hz, maybe 120??

put larger subwoofer in corner to handle the sub 50 hz stuff, maybe a matching one in opposite corner (maybe the upcoming FF5?)

Room is 17x30x9. Concerned that the supersats will be thin in the mid bass and that the room is large enough so that the FF3s may not be strong enough to handle the upper and lower bass at the same time.

That works - running speaker wires through sub to L/R Sat's, set to small with 50Hz crossover, crossover control on sub will determine sub's low pass point, high pass to sats is pre-set, then adding in-room big sub for bass below cutoff. Only change I might suggest is a higher low pass/higher pass (60-80Hz) so the in-room big sub gets more activity. Center Crossover set to 120Hz so big sub should be in the front of the room. Alternative would be L/R as "Large" so big sub would be LFE-only and could be placed anywhere.

how would the sound of on wall plus mid bass differ from a tower in a cabinet out from the wall? Will it sound thin due to lack of cabinet resonance? Or from lack of position out into the room?

Not exactly sure what you're asking - how would SuperSats mounted on the wall, used with a ForceField subwoofer, compare to a Triton Two or Three? If you add a second sub, pretty darn close.

Again, not sure what you are asking about cabinet resonance and sounding thin. The SuperSats are designed for on-wall use, the frequencies reinforced in an on-wall application are below the usable frequency range of the SuperSat (below the appropriate crossover). Positioning the speakers (any speaker) out into the room a bit will improve the three-dimensionality of the sound. Since on-wall and in-wall is a fairly common approach these days, this issue is obviously not a deal-breaker for most listeners.

Hope this helps.