Max recommended voltage for a Core i7 920 is 1.375. Here is the link to Intels Processor Finder site: Intel® Core™ i7 Processor I7-920 - SLBCH. You would be fine using this, and to be honest Intel don't have any way to tell what voltage you have set anyway, should you ever need to RMA the CPU.

There isn't much difference, but as I say, running with LLC enabled is out of spec. vDroop is there to insure that when you set vCore in the BIOS, it never goes above that value. With LLC enabled it will go above that value, frequently. It's logical to worry about going out of recommended vCore, but it's worth just as much consideration as to if LLC should be used. Both scenarios will mean you are operating your CPU in a way contradicting recommendations.

The Anandtech article gives a good explanation. If you have read it all the way through and still want to enable it, that's fine. It's better though to make an informed decision, even if it won't do any damage.

For instance, Anandtech found that although they could set the vCore to less, the actual minimum load voltage needed to be stable increased with LLC on, and power usage (and probably heat generated) went up, by a not-insignificant amount.