Having spent several years of my professional life working on manual gearchanges I immediatly noticed the notchiness and the kick back especially 2nd to 3rd. This feeling of the gearbox having a little kick back at you is called double bump in the trade and people have spent many years playing aorund with it... but without boring you too much, its a function of synchroniser performance, friction and shift lever ratio amongst others. The more direct and low ratio the shifting, the more likely you are to notice.
The 159 seems to have a very direct low ratio shifter, which provides a positive short throw 'sporty' shift but also is then prone to notchiness and kick back etc - if there was lots of rubber in the linkage joints you would feel less notchiness but shifts would be a lot more vague. Go drive an early k series lotus elise for an example of this in a sports car!.
Oil is by far the easiest starting point and i notice some people have had success with Castrol SMX-S. This is a first rate oil and will almost certainly improve synchro performance, although may have an effect on the q2 diff but not too likely to be noticeable.
Redline oils can also work well for transaxles.
Think i'll be adding oil change to the list for the next service as i'm not too enamoured with the shift quality. The other option is to graft a wacking great extra counterweight on the box lever to smooth it out further but the car is probably heavy enough already!