Now you are contemplating a step colder by putting ik20,manual says ik16 from denso and bkr6e from ngk,electrically conductive deposits may form on the insulator (fouling) causing a loss of spark energy or the actual shorting-out of the spark current. Idling is affected, and in the long run, misfires will happen and of course loss of power but if you can take very good care of your vehicle it might be ok.
A colder plug like IK20 might be good for continuous long runs, but not so good for short distance travels.One of the reasons why there is a heat rating is that the plugs need to be "self-cleaned" by incineration. The deposits get burned away with higher heat. Of course you can't have it too high because there will reduce plug life and performance. Higher heat rating plugs conduct heat more efficiently to the coolant system.If your engine runs very hot due to high combustion temperatures , then you will have to take higher heat rating plugs. IK16 is hotter than IK20 means, heat accumulated at IK16 is less dissipated than at IK20.
With IK20, probability of misfire is alittle higher as mostly our throttle is only between 1500-3000 and rarely need to reach 4000, hence, may have less heat for 100% self-cleaning and affirmative firing. For city driving, and short trips 16 would be better and also recommended by the manufacturer.