400mm on the NX200 (many big pix!)
I don't think it's heavy enough to break the lens mount, but for usability a tripod collar is a must. I haven't find a suitable one yet, probably will have to modify a standard one.
Not too bad! However, the value of the TC is further diminished by atmospheric haze - avoidable mostly Fall and Winter.
"The whole system is extremely heavy, unusable handheld and hardly usable on a tripod." Note of caution: Such rig may require a lens tripod mount (or ring), to avoid failure and loss. The moment of a large lens (torque) at the NX/NEX adapter can break it open. http://www.grc.nasa.gov/WWW/k-12/airplane/torque.html
With rare exceptions, 99% of all the NX and NEX glass adapters use only 3 tiny screws to attach the adapter ring (legacy mount specific) to the adapter body (common for multiple legacy mounts).
Worse, these 3 screws often are wrongly placed: Only one up, where the moment arm pulls the screw, and two towards the base (only compression acts here). Engineering common sense --which is abundant in China-- would reverse this, and place 2 screws up at 10 and 2 o'clock, and one at 6. Or better, always use 4 larger screws on the NX and NEX adapters (a rarity, i just ordered one).
DR: I'm currently comparing NX's SmartRange vs. NEX's DRO. http://forums.dpreview.com/forums/readflat.asp?forum=1001&message=42624919
The NX100 doesn't look too bad in DR, though its sensor doesn't extend as far and smooth (roll-off) into highlights as the Exmors. Mitch