One factor I haven't noticed in this discussion is the condition of the course. If the entire course were water injected such that it would support your weight without a shorter ski digging in excessively anymore than the longer ski and thus slowing you down I can't see why a very short pair of skis wouldn't be superior provided you can maintain good fore aft balance. I have a pair of 165cm race stock Elan SLX that I like very much and I have found myself suddenly behind the skis on several occasions. It really takes quite a bit of skill to stay centered on them. Unlike the older "straight" slalom skis of an earlier era you cannot get momentarily back on the skis and depend upon them to support you while you play catch up to regain balance. I mean if you aren't sinking into the snow and the amount of edge you have available is sufficient to hold an arc then you might think a pair of ice skates would be ideal. I'm being a bit facetious here because even water injected courses aren't usually hard enough to keep an ice skate from sinking in but I imaging you can see my point. The biggest drawback to a shorter pair of skis that I can think of is the amount of skill required to keep you perfectly balanced over them at all times on a course. Since the acceleration of the skis varies considerably depending upon changes in slope, snow surface, radius of the turn and position in the turn you must anticipate all those factors in order to have your body in just the right place at all times. Longer skis provide for a greater margin for error.
This is all based upon the assumption the skis shape ie turn radius is appropriate to the course. In the old days ski shape in a turn was much more a function of the flex of the ski and the skier's ability to shape the ski accordingly. Thus a 200 or 205cm slalom ski was pretty much the norm. That was before the advent of water injection allowed for shorter skis. Even the longer skis would produce fearsome ruts for those out of the first seed in the softer snow conditions.