Don't the 505's do gate starts? I read that this has worked well in their big (100+) fleet championships.
Contrast with the first beat in a Laser World Championship which is about 1 mile. For the same number of boats as a 5o5 fleet the gate would be approximately the same length which would be a significant fraction of the weather leg distance. Then think of a more typical Laser regatta where the first beat might be .75 miles... The issue here is that where you start in the gate can change the distance sailed by a significant fraction, which is much less an issue for the 5o5's with a longer first beat.
As I recall, a gate start was tried at the '99 Laser NA's and it was not considered a success.
Anyway, it seems that in major Laser regattas the solution is to split the fleets and try to keep the start line down to the range of 50 boats or so.