Stoneysteve,
I have used the OMS IQ harness together with the 32lb single tank travel wing, OMS no sag weight system and steel cylinders up to 18L without any issues. The IQ harness is well made, and unlike the Dive Rite Trans Pac or Oxycheq A Pac, the IQ harness has a padded back support concealing a sleeve for a backplate to be inserted and then secured with a velcro flap system. The interwoven nylon material is quite substantial, and I would imagine it would take considerable punishment.
For diving Czech lakes in winter yes I used the hefty OMS STA and SS backplate if for no other reason than to get some weight of my belt, and yes the system configured like this is rock solid. When using the IQ harness with the tank secured to the wing/harness via just the cam bands then you can detect some movement, especially if you're used to diving with a BP/W, but its not much to speak about. For warm water, resort type diving then the IQ without the backplate is fine. OMS themselves suggest that a backplate be used for doubles:
http://www.omsdive-uk.com/backplate.html
A dive buddy has a Deepoutdoors matrix BC, this is a similar idea to the IQ harness as it incorporates a pocket for an optional backplate. He's had no issues with the larger steel tanks as well.
http://www.deepoutdoors.com/matrix.htm
The great advantage to the IQ harness is that you can slide in a BP for local diving, then remove the BP when you go on overseas dive trips. All the current OMS wings have cam band slots, however if you use the IQ harness with a backplate and wish to thread the cam bands through both sets of vertical slots in the BP, then you might have a tough time unless you've got small and dextrous fingers! I could only ever manage the lower set of camband slots! I may swap and change my kit every couple of weeks, but the IQ pack is pretty versatile and I've held onto it.
Regards,
Charles