i m afraid that if i put 16"' i will lose some of the acceleration... what should i do?
typically, a bigger wheel like a 16" is heavier than the smaller wheel; and the tyre is only marginally lighter but not enough to compensate for the heavier wheel.
so you WILL lose some acceleration speed. however, it may not be very noticable, so a 16" wheel should be ok. the other 'con' is the heavier wheel-tyre combo increases 'unsprung weight', which you may have heard about. UW is the amount of weight that the suspension (spring+damper+antiroll bar) have to contend with. the heavier it is, the more they have to 'work', and the harder their job, so the less effective they are (without modification). basically, you will lose handling/ride comfort too for this reason. BUT again, the difference may be very marginal when going to a 16.
things become 'worse' with bigger wheels like 17s and 18s and of course 19s. but people can go to some effort to minimise the adverse effects by:
*buying lightweight wheels: can be expensive, hard to find the correct fitment for an alfa, and not be as strong and therefore more prone to damage/buckling
*upgrade dampers, springs, ARBs
16s and 17s are probably the best size in terms of compromising between handling and ride comfort. i think if you change from 15s to 16s you'll appreciate the improved cosmetics and handling, and not really notice any reduction in acceleration or ride comfort; not enough to worry about.