Any significant chance of bricking devices with this mod? I sent 30 units to be moded and 6 of them came back "dead" 
That is 20%! Basically they appear to be hashing in cgminer but no shares are submitted at all. All 6 units were working fine at 850 Mhz before the mod. If anybody experienced something like this before, please let me know what you did to solve it.
Yes, I had the same problem. Turns out i didn't have enough power to the units. When modded they suck more juice! Take 10 of them and move them to another power supply, I bet they start working then. That is 20%! Basically they appear to be hashing in cgminer but no shares are submitted at all. All 6 units were working fine at 850 Mhz before the mod. If anybody experienced something like this before, please let me know what you did to solve it.It is not a power issue. I am using an ATX PSU with 4 units per PCI-e line. Also tried them individually with a dedicated 6A 12V power supply. I am also thinking it must be either the soldering or maybe bad SMDs or something. One unit is showing short-circuit signs with "spark" noise and the ref light flashing really fast. My question is, even if it is bad soldering, are there any changes of getting them to work by redoing the mod? Or are they completely dead now?
One thing I did notice, the damaged units only eat about half watts as the good ones (8-9 watts at the wall while the moded ones are a bit over 15 watts)
Try running the "damaged" units at a much lower freq, like 600. Most likely either the solder joints are crap and can't carry the current or the person really messed up and just flat out made it a bridge by using too much solder.
In each of the few cases I have fixed a botched solder job (either by myself or someone else) on a gridseed, the unit was back to operating as expected after the fix. Really depends on the degree of failure but if the person just messed up soldering only that SMD then chances are the gridseed is just fine.