I'm not an experienced slin user but I'll try my best to answer your questions.
Do you become a diabetic by using slin? Not likely. Although some have suggested that long term use can lead to diabetes in the future.
Predicting the gains on slin is near impossible. It entirely depends on how good your diet and training are. Slin is a shuttle really that gets nutrients into body cells so that they are available for repair and growth. If the nutrients are available and correct training has lead to the need for them then slin can work wonders.
Coming off? Just stop taking it. Simple as that.