I would be curious to know just how this 'skeg" is attached, and where.
The rudders on the boats were not particulary troublesome-A small percentage suffered from quality problems, not from design flaws but rather the "Friday lay up" syndrome. This could occur with any boat.
As to the pluses and minuses, the boat was not designed with one, and does not really need one. Certainly it would slow the boat down due to the extra wetted surface area.
The only benefit, if you want to call it that, would be some protection from objects strilikng the rudder when underway. On the other hand, the shape of the hull with the longish fin keel will probably do just as good a job.
Like the 32 and 35 (70's vintage models), the rudder is a tad small, and if one were looking for real improvement in this area, a deeper, higher aspect rudder would be the call. More positive control and better upwind performance would be the result.
Bottom line: I would be leery of a mod like this and would need to know a lot more to offer a more specific opinion.
Good luck!
Seth