Ideally the Uncore ratio should be the same as the core ratio (as it is at stock values) or (particularly in the case of overclocking) higher than the clock ratio to not constitute a bottleneck. On my Intel i5 6600K I set the Core at 46 (4.6 GHz) and the Uncore at 49 (4.9 GHz) - yes, much higher than the Core! I tested it stable on a 2H Prime95 Blend test up to Uncore 52 and could probably go beyond that value but the overall performance drops after 49, so that's what I use.
To measure performance I measure the time it takes for the system to complete the Prime95 800K test on the four workers. Setting the Uncore ratio at 49 rather than 40 shortens the time by some 7 minutes (from 48 min. to 41 min.). Getting 7 minutes shaved off on 48 is getting 14.5% of raw processing (computational) performance gain! It does not change the Temp. and Power significantly from 40 to 49 (topping at 84 C max under 26 C ambient and 104.5 W) although I had to up VCore from 1.355 V to 1.390 V to ensure stability. The stability of the PC (BSODs, hanging, etc.) is also much greater, in fact rock solid (never had any trouble ever since). It is so good that I really feel it.
Games that would usually get the 4 cores at (for example) 98-96-100-97%, now leave them at 83-87-93-84%, further showing the overall gain in CPU performance. THIS IS ABSOLUTELY FANTASTIC !!!