I'm running just under 4.2GHz on air cooling right now. My CPU does seem to need more VCore than yours does. I have to use 1.475v and not an increment less. I'm not sure what you mean by the CPU hold down issues. I haven't heard of any issues with the CPU socket which is what I believe you are referring to.
Yes, the socket.....I had a brain freeze...........a guy on the EVGA forums, who is quite experienced, went to change out his 920 C0 for a D0 and lost a handful of pins. He had some pretty good photos.
I don't think there has been any feedback on what the result was, but he didn't seem to have mounted the original incorrectly...he simply opened the hold-down plate, removed the CPU and a dozen or so pins came along for the ride.
Needless to say..........RMA.
On another note.........my CPU seems to have hit the wall where I am. I've tried to increase the VCore and VTT to be able to increase the BClk, but I think I've gone as far as my CPU will let me............I know the board has more in it........but I can't see replacing the CPU just to gain a few MHz.