Hi Darryl,
We sail a simular fromat but start the other way round.
1st less than 16 foot
2nd 16 to > 18 foot
3rd 18 plus. including 16 foot + spinnaker boats.
We mainly race back to back short course races of about 45 min each. Starting the slower boats of first gives them a start on the faster boat. That way we get the fleet to finish closer together and can get the next race on the way sooner. Another thing it does is eliminate the likleyhood of the fast boats (spinnaker) coming down to the bottom gate near the start as another fleet is starting. Very scary in big breeze going head to head with boats just after a start. Not many gaps

At the larger mixed fleet regatta such as Forster Wild Cat we had a start F-18s (Their National title), followed buy H16 and smaller boats and last.... The rest of the fleet. On our start we had 70+ boats. This is what racing is all about.
The first race off the regatta we pulled a port start on a fairly neutral start and only just cut in front of the A's and other T's. Right was favouring. The second race we lined up again on port and made it quiet obvious. Ashby and Brewin shot down to the pin to teach us a lesson. We ducked them and were forced to slice through the middle of the fleet. Interesting to say the least

. Anyway right payed off again and was 2nd to the top.
All this was done at the pin end where Australian Sailing were photographing

. Twice was enough and we started the rest of the regatta on starboard.