3.For the most part, D0s overclock higher and at lower voltages than c0 chips, due to refinements in the fabrication process. Late c0 chips benefitted from the same refinements, and random other batches hit 4.0 ghz at really low volts. I have been told that some d0s do not hit 4.0, but most will do so at a lower voltage than most C0s. A d0 is a more desirable chip, but it's not a must and this guide is still useful to owners of C0s; I have owned both. There are definitely c0s that are better than d0s.
4.The toms article about speed vs power consumption is flawed and based on one really terrible c0 that needed 1.5 vcore to hit 4ghz. Is a d0 at 1.2v at 4 ghz going to consume more power than a c0 at 3.6 and 1.32 volts? No. The d0 will use less. Power consumption at any given voltage will increase about 3-11 watts (avg 5 for every 100 mhz you increase). Something brough to my attention though recently is that this may differ because some chips are leakier than others. A d0 is by no means a guarantee that it will consume less power.