possible upgrades for merida speeder 200
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.