Software voltage control was part of the Core 2 design. When Intel released the first generation Core i, simple voltage control was removed. I think it was not until the 4th Gen Haswell that voltage control returned as part of their Fully Integrated Voltage Regulator (FIVR).
When you go into the bios on a Dell T3500, there is no option available to increase the voltage going to the CPU. The EVGA Classified3 X58 board has a ridiculous number of voltage control options so you can finely adjust the voltage going to every part of the CPU. Luckily you can leave most of these voltage settings on AUTO and still get great results.
When you raise the TDP/TDC say to 200W, does it raise the Voltage to support the OC on the unlocked Xeons as long as temperature is within limits?
Adjusting TDP or TDC does not have any effect on voltage. These 2 values determine when the CPU will start to throttle (reduce) the amount of turbo boost being used. TDP controls power in Watts and TDC controls the amount of current that can flow through the CPU in amps. The 4th Gen and newer CPUs calculate an estimated power consumption value internally and then this info is used to control how much turbo boost the CPU is allowed to use. When power consumption exceeds the set TDP value, the CPU will slow down just enough to keep the calculated TDP under this limit. I will post some pics later of my overclocked W3680 to show what happens to the MHz when it starts hitting the default 130W TDP value. These power values can be increased on the unlocked W3680 and W3690.
For the 1st Gen Core i, there is no publicly documented way to report power consumption. This data is generated internally within the CPU but no way to know exactly what it is. It is more of a guessing game. If you set the TDP to 130W and the CPU is not reaching full speed then it must be throttling. If you increase the TDP and it starts running faster, then you know it must have been hitting the TDP limit. Some of the Asus X58 boards were running into throttling problems because one of these limits was not set high enough for extreme overclockers. Asus ended up releasing some special bios versions under the table to keep the overclockers happy but they never offered these on their website.
Or is the BIOS locked at the default Voltage under all conditions?
CPUs use a voltage look up table. The CPU determines what speed / load / temperature it is running at and then asks the motherboard to give it the appropriate amount of voltage so it can run stable. Intel CPUs always ask for a little extra voltage so the CPU will continue to run stable well after the warranty is over. This extra voltage allows the CPU to run stable even when it is overclocked a little. On the 6 core Xeon W3600 series, the amount of voltage needed really starts to go up significantly when you go beyond 4 GHz. On a board with no voltage control, the built in VID voltage table only seems to have values good enough to run stable at 4 GHz and not much beyond. The Intel specs list the VID Voltage range for all of these processors as 0.800V - 1.375V. I think my Xeon W3670 needs approximately 1.375V to run stable at 4 GHz. Without adjustable voltage, that is approximately where you will end up.
There is no real difference between the Xeon W series or the Core i7-990X Extreme. It is all luck of the draw. Maybe a CPU with a high VID table will give the CPU a little more voltage when used in a Dell T3500 and this will allow slightly more overclocking or maybe a CPU with a low VID voltage table will allow a user to overclock more because it is a great overclocker without needing a lot of voltage. That might be perfect for a board that does not offer voltage adjustment. The Xeon W3680 does just fine on a voltage locked or voltage unlocked board. Definitely not worth paying 2 or 3 times as much to get an Extreme Core i7.
I have seen a hard mod before and some great results when feeding a 1st Gen Core i7-920XM Extreme mobile CPU some extra voltage. I think it was svl7 on Tech Inferno that did this. Unfortunately every time I try to go to their website, my antivirus software warns me about a Miner virus so I would not recommend going there.