Every little add on will shave a few °C.
Delidding itself lowers it most when removing the stock glue (-10°C to -20°C)
Adding LM instead of regular TIM when relidding shaves a few more (~-5°C)
Putting a copper IHS with bigger surface area is a few more (2°C to 3°C)
Basically, just the delid itself and relid with regular TIM does the most part. The problem with the CPUs, contrary to what almost everything says on the internet, isn't the TIM used, but the gap between the CPU die and IHS. The black silicon glue Intel uses creates a gap between these and causes poor contact. So just delidding, removing the glue, applying standard TIM and relidding will save you 10 to 20C by itself.
What determines how much you save is entirely specific to your CPU, not the model, your specific CPU. As you might know, all CPUs aren't build exactly the same on a nano level, this is why some CPUs clock higher than others and even same clocking chips with same voltage won't output the same heat. The best chips are usually the ones closer to the center of the chip wafer. For example, my 8600K requires 1.4X volts for 5.2 GHz, but requires nothing but a intel stock cooler for that AND doesn't go over 75°C, because it's a low leakage chip, extremely efficient, but not a great overclocker (also the kind of bin for SFF rigs). Higher leakage chips, usually the ones with the highest temperature drop with delidding are the hottest and clock really, really well.
Give this a read, very informative, written by a friend of mine :
https://www.techpowerup.com/reviews/Intel/Delidding_the_Intel_Core_i7_7700K/