Boyer Mk IV and better would be more than competitive in a mixed fleet. That includes the Flyer I which just was third in the Worlds. The important things are that the platform is stiff and the rig/sail is up to date. Any of the designs you will find from the mid nineties in AUS will be highly competitive. It is mostly the skills of the sailor that will get any of those boats near the front of a mixed fleet on yardstick. I'm not sure which A Class legends have been telling you the 90kg BS. There are plenty around that weight and more who are highly competitive in the national fleet. Don't use it as an excuse. "A" rigs are highly tuneable to suit different weights and can be powered up or down easily.
Go for it - there is a national title in your State in 6 months time.

Cheers
AUS9