I'd go up 2 in the rear or down 1 in the front. Up 3 in the rear is possible and is about the same as one down in front but shortens the wheelbase enough that it is noticably lighter and more wheely happy. No more than this is needed to make take-offs effortless and more compromises the bikes' all around abilities. Riders making more drastic changes usually have a specific gearing goal in mind for track purposes.