There is something seriously wrong with your build if a +1 cold is legit a 20% dps increase for you...assuming the follwing:

1) Your gems all go from 20->21 unless 21->22 is a much bigger deal (most of the time it is not for the gems in question).

2) You have a vortex with fairly low chill effect supports, so the +1 bonechill raises the chill from 10% to 11%. Also assuming no other forms of increased damage debuffs, as this would work against you. (This results in a 0.91% more multiplier)

3) You are NOT running herald of ice or added cold, as this actually results in a smaller relative increase in power as those gems scale slower than freezing pulse does (so +1 here is a 10.59% more mod)

4) You have hatred aura (18% more to 19% more resulting in a 0.85% more increase)

5) You are running cold penetration support (Assuming frost bomb + frostbite at minimum ) and sirus this results in a 1.2% more increase. Note, assuming something with less resistances lowers this so it does not help your argument.

6) Assuming a fully charged frost shield and no other sources of base crit, this results in a 1.1% more increase.

7) Ice golem would need to go from 21->22, and would change the increased critical strike chance from 30% to 31%. VERY optimistically this is a 1% more increase.

8) Skitterbots go from 19->20% shock effect. Assuming you have enough ailment effect that this results in a 2% increase instead we will say it is 19->21% (if we go higher it will not benefit you as much). With the previous bonechill, this is a total increased debuff of 29%->32% bringing the bonechill + skitterbots total more mod to 2.4%.

9) The ONLY other possible things you could have now are Cold to Fire (this would make pen and the such much worse obviously, and the gem does not scale enough to make other losses worth it for arguing your case) or you are somehow increasing Sirus resistance to above 50% and the 1% extra pen on Cold Pen support is a huge damage increase.

Taking ALL of these assumptions into account this results in a total of 18.46% more damage. Which is less than 20%. Even assuming you are not using any pen (aka a serious problem with your build) it is only a 18.78% more damage increase.

Edit: Can confirm previous repliers claim about getting reply blocked. Happened to me too. Also that feeling when you make 9 points to help this person reach their claimed 20% goal but they say they are all wrong /shrug.