Yes I did try this on my old wheels. They were 6.5J, I think the pepperpot 15's come in two sizes 6J fitted on the 1.6 and 1.8's and the 6.5J's on the 2litre and V6 models. Then for some strange reason Alfa fitted 185/65 15's to all model except the V6 were they fitted the 205's.
You benefit from more grip, but you notice an increase in tyre squeel as you hurtal it round the bends. If you're looking for more grip go for it.