It varies per individual part and undervolting is usually done for a given clock speed, not so you can raise clock speed.
You can either start by undervolting and seeing what your lowest stable settings are, and then see if anything from the stock voltage to your lowest stable voltage allow you to overclock a little bit or you can go the other way round. So, instead of lowering voltage, raise clocks to see what your highest stable clock is, and then start tinkering with the voltage.
If your clock speed is not limited by voltage, or you just have a very good chip, you may be able to undervolt and overclock, but I wouldn't get high hopes for very significant overclocks.
I tried it. Worked. Got my 1050 to 1950 mhz on core clocks and 4200 mhz on memory clocks. I used MSI afterburner and adjusted the voltage frequency curve to make the GPU hit 1950 mhz on 950 mV and 4200 mhz on memory.
Also how do I add a screenshot from my desktop to tom's hardware?