I don't like the coopers ginger beer - too much artificial sweetner that leaves an aftertaste. having said that, i reckon it's best to make it up just as per instructions, however when bottling use some Buderum Ginger Refresher instead of sugar.
personally i think the carbonation should be fairly high for GB. If you don't want it alcoholic, i've been told you can just add the tin with water up to 18L and carbonate, and it tastes good. Never done it though.
I've done a few of these and yes the sweetness hangs around .
I used a packet of beer emzime in the last brew (2 tins 40lt 3kg dex 100g fresh ginger) and this is much better.
Want to try a morgans next as its been said here they are not so sweet..