I am looking for a low wattage CPU with relative high performance. I read on several places, that underclocking or undervolting cheaper normal processors (e.g. Xeon e3-1230v3) can have the same result as buying a rare and more expensive L or T version (e.g. Xeon e3-1230Lv3) of the same processor. Is this true, or should I buy the more expensive low wattage version?
The answer is that it depends. If the one processor is faster than another, you can definitely undervolt the processor, or underclock it to slow it down. It would be dependent on how much you undervolt or underclock the processor, however. You might also want to note that you would want to make sure that your motherboard can support overclocking so you can ensure that you can actually mess around with the processor states and voltages.
Also note that the power consumption scales linearly with frequency and quadratically with voltage. So you can also reduce the frequency on a cpu state to lower power consumption as well, but it is more effective to decrease voltage because it has a squared relationship with power consumption.