The correct answer to that question goes as follows :

When the total amount of sailarea is limited to a relatively small amount then it is most efficient to put it all in a mainsail of sufficiently high aspect ratio.

When the area in the mainsail is limited by itself (for some reason) but a higher total area is allowed then a mainsail + jib combo is quicker overall and often also upwind when the jib area is at least 20 % of the total. You point lower but go disproportionally faster.

When spinnakers come into play then having a jib is less advantagious then before as the true power of the jib lays on the reaching legs and downwind legs. Of course the spi will then take care of the downwind part leaving only the reaching parts and some additional performance on the upwind.

There are a few reports of spinnaker boats of identical make racing eachother where one is sloop rig and the other only has a main sail and still the sloop rig has better upwind performance although relatively minor.

So the whole point comes down whether you can add a jib to the orginal sized mainsail (=faster) or whether you have to take the jib area out of the mainsail (=slower).

The A-cat class rules are of the latter type and therefor you will not see any jibs on A-cats anymore. However, the F16 rules are of the first type and here the added performance of the jib is used to ofset the performance hit experienced by having another person on board. Overall it appears the F16's are pretty equal around the course when comparing both modes (1-up uni and 2-up sloop) with the sloops being slightly faster upwind and the uni's being faster downwind.

Wouter