You seem to be overlooking one small fact Wouter when comparing uni with sloop (same boats) and that is that most (not all of course) cats when sailing sloop rigged sail with two people on board but usually only with one person on board when uni, this weight difference obviously will have a strong bearing on the comparitive performances.
The most obvious example of this in Australia where the uni/sloop contest has been in hot competition for over 30 years between the sloop and the cat rigged mosquitoes. The results have varied over the years when for some time the uni's have been superiour and for other years the sloop have had the ascendency. About the only real conclusion that could be drawn over such a long period of time is that the only real difference has been the quality of the sailing skills of the people in the relative classes at the time and that in either form the boats were too close in performance to be able to make any truely OBJECTIVE conclusion.