How difficult is it to drill a HD650 cable to get the connectors

Not worth the trouble, I'd say. Just save your HD650 cable and buy the connectors for the peace in mind.

Cutting apart the cable to get at the connectors shouldn't be that difficult but I am pretty sure the wire is enameled.

The driver uses some like 40+ awg cable I would be afraid of breaking it personally :O

I am totally confused about the drilling a 650 though :O  It's like a small tiny wire connected to a slightly thicker coil that is held over the hole where the plug goes into.  I would be terrified soldering something to there to use as a permanent cable...  Stepping on the cord would have a good chance of breaking either or both drivers if the cables were soldered imo.

Another recommendation to get the DIY connectors, they're fairly inexpensive ($16/pr), and a lot nicer to work with than butchering the stock ones.

Cardas 650 plugs cost about the same as a 650 cable. 

On that note, people have had great success with "partial recables" where the cable is cut a few inches from the connector, and wires spliced on from that point to the main plug. 

People have also soldered the new cables directly to the driver. Its a bit of a PITA, as the wire is very thin as pointed out. To make matters worse you can only screw it up so many times before you cant salvage the drivers.

As another note the wire in the 650 drivers feels more brittle than the copper magnet wire I used, and the leads on a fubared before I got them set of 650 drivers I bought crumbled in my hands. Im pretty sure that I have read that the voice coil on the 650 is wound with aluminum wire rather than copper to save weight. There is a chance Im wrong, so dont quote me....

I have made 40ga magnet wire interconnects (they were surprisingly durable and lasted just under 2 years including travel to countless meets) and the wire does not break like the 650 voice coil wires.