wouter,

in most cases, adding sailarea makes a boat faster.

if you put that extra sailarea you propose to put into a small jib mounted low to the platform, into the mainsail (e.x. higher mast), I doubt the sloop will be faster than the uni in light air. light air is interesting because a boat and sail can be sailed to it's full potential. A and C class both allow you to sail with a jib, still even 60 kg light A cat sailors who might start pulling cunningham at 2bft are not doing it. in contrast 'heavy weather' boats like the F18 seriously struggle in light winds. If class restrictions would allow them to put the sailarea of the jib into the main (with a longer mast) my guess (no science) is it would perform better than the one with a jib till the moment the heeling moment can no longer be compensated by the rightning moment. by the nature of the rule the uni will be earlier overpowered than the sloop as it will be earlier lacking power in light airs. the reason why so many (succesful) boats and classes have jibs is to make them suitable for a wider range of conditions by giving up light air performance same time. I assume the AM2 catamarans on the Swiss lakes are a good example... their top mast gennaker are cut so flat they seemed to be usable upwind. saying this, if you want a good alrounder, a sloop offers a wider range (something you need on a RTW) while if you design for a particular small windrange, a uni wing will probably perform better.

by the way I would be highly worried if by my next step on an airplane I would notice jibs in front of the wings ;-)