Tiller extensions are not allowed and for a very good reason. Nor are any lines, bungees, or the like. You have to steer with your hands, feet or whatever other body part you feel will work. Hummm, that leaves a lot to the imagination.
After 14 years of racing them we have all concluded that lighter sailors are faster off the wind. But, once we found that the taller, heavier people could get further forward than the little people, they did not gain as much distance. If they had extensions, you would automatically lose all bigger sailors in the class.
There may be many other factors to your problems. Several of us tried Whirlwind and made suggestions to Chip to get them faster, but he didn’t particularly want to work with to get them faster. For your weight you will need a fuller sail than your smaller competitors. At this point the best sailmakers are Calvert (sold on this site) and Bossset North Sails (NJ somewhere). I am playing with a Dacron Calvert that is very full – I was 235, but now down to 192.
Next there is the rigging. The hot setup is to use a 10 hole adjuster on the forestay and have slack sidestays. Mine were so slack I took the adjusters off and just use shackles.
As you found out weight distribution is paramount.