I have a Speeder 200. (2015). I want to upgrade it's rear derailleur, my problem is I am unsatisfied with its performance. (it is a Tiagra with 9 speed at the moment). If i replace it with a 105 (9 speed) should I replace the other components(cassette, chain, front derailleur etc.) as well? Or can i replace with a ultegra?(does a 9 speed exists) Should I buy a groupset? haven't tried an upgrade before.
edit: I have been using it for around 400 kilometers, components are still fine, but I was never happy with that Tiagra. I have some problems with shifting, if speed is >20km/h it does not shift properly. I have taken my bike to Merida service twice, I don't think there is a cabling or another problem. It seems OK, but on road i dont like its poor shifting. For example I change it 6th to 7th, after a few cycles I hear a mechanical sound and it decides to change, not on time.
I had another bike with Deore rear derailleur before, never had an issue like this.