Is there a difference if we compare the classical cooling of the whole keg:
and the quick flow cooling where the keg is at the ambient temperature?
Can the quick cooling have negative effect on the beer's taste or head?
I can think of three possible impacts this scheme might have on a keg of beer.
As long the beer's sealed, there should be no difference in taste due to rapid or slow cooling. Lowering the temperature only increases the solubility of CO2, which should dissolve later.
Temperature itself does impact taste, supposedly due to our taste buds being number in cold, hiding certain flavors (which can be desirable or undesirable depending on the beer). But this isn't related to the rate of cooling of beer.
In short, no. The time it takes to cool a beer will not have an impact on the flavor of a beer. However there are certain temperatures that are preferred for serving certain beers.
The most damaging thing to beer flavor is light pollution. When in direct light, isohumulones from the bittering agents in hops can react with riboflavin (Vitamin B2) in beer and produce 3-methylbut-2-ene-1-thiol (MBT) which has a noticeable skunky flavor and aroma.