I read elsewhere that redline gearbox oil can fix this.

Ive been having the same issue from 2nd to third when im shift over 4k.
apparently the oil is lightweight and gets between the sychros and stops them from crunching on shifts, how true that is im not sure but im gonna try it out for myself next week.
link here http://www.wrx.com.au/forum/viewtopi...=5&f=6&t=35370

Cheers

Will