I generally use about 20-30 habanero or scotch bonnets to make 2 x 150ml bottles of sauce. It is very hot, but retains a good fruity taste.
Also, I use about half a sweet pepper to bulk it out a bit, rather than tomatoes as they tend to add a bitter edge which is why I suspect there is so much sugar in that recipe. Many seem recommend trying rice vinegar too, as it has a much softer and sweeter flavour than wine vinegar and may help you to reduce the sugar even further.