I did the upgrade because in my opinion the skimmer was good, but the Tunze 9002 is better. The stock skimmer is VERY sensitive to your water level. When I got the level right and added my ATO the skimmer seemed to work a lot better.
Also, before I did the upgrade, I did some modifications to the original skimmer to make it quieter and more efficient. It worked good, but I feel the tunze does a better job. The In Tank media rack makes maintenance much easier as well.
For the record, the original skimmer worked pretty good.(a little noisy)
To answer your question. The skimmer does not need to be upgraded in my opinion, but the upgrade does make it easier and the Tunze is a little more efficient.