No arguing that drag will increase with slots, which is why I've only really imagined them in the downwind dash scenario. There we are operating at as high as angle of attack as we can , proabaly well past most efficent. I wonder whether by running deeper, if a little slower, with slots (ie closer to the spin equipped boats) it would be possible to make more VMG. For general aviation take off/ landing, drag is probably pretty irrelevent to the designer. However if the goal is different then the placement and sizing of the slots would differ to meet the changed target. The only windtunnel work I know of in this area has been done for paragliders, and is shown on
According to independant tests (in the table 2/3 the way down) they got virtually 20% decrease in stall speed (ie high angle of attack) for 2% increase in drag at minimum angle of attack (equivalent to upwind). Certainly those things sell well enough. I've spoken to owners and they say the only disadvantage is a narrow 'poor performance window' at medium angle of attack - might be a problem on a sail for beam reaching, but not up/downwind. Just makes me wonder - but we don't see so much experimentation nowdays with rigs, that all seems to be on the kites.
Regards the 380 the big plus for me would be the width. Once used to wider tails I found going back on a raceboard takes quite a readjustment of style.