possible upgrades for merida speeder 200 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.
All Shimano road derailleurs (2400, Claris, Tiagra, 105, Ultegra, DuraAce) made for 7-10 speeds are compatible with eachother. The amount that the derailleur moves is determined by the shifter.  With 11 speed they changed things up, so as long as the derailleur you chose was designed to work with 7-10 speeds you should be fine. For mountain derailleurs (Acera, Alivio, Deore) 7-9 speed derailleurs are compatible with the 7-10 road groupsets. 

A 9 speed Ultegra or even DuraAce can be found at 9 speeds if you find old enough equipment, but all current 105, Ultegra, and DuraAce are 11 speed and will therefore not be compatible with your current 9 speed setup.  If you want to find a 105  9 or 10 speed derailleur, it won't be the current model, although the 10 speed 105 (5700), Ultegra (6700), or DuraAce (7900) is still readily available as a replacement part. 

As an aside, are you sure that upgrading the derailleur will really fix your problem? What kind of problems are you having with the Tiagra? Could it be an adjustment or bad cabling that's causing your shifting problems?  Is your cassette or chain worn? 9 speed Tiagra is older equipment and it's possible that your chain or cassette are at the point where they should be replaced. I have 2400 (8 speed) level equipment, and find that it shifts quite well as long as the adjustment is good, and I usually don't need to adjust it very often.