Here's my commuter bike that I was talking about
:
It's a GT Zaskar Urban Machine commuter bike, and it came stock with an EF50 8-speed Shimano rapid-fire shifter, and Shimano Alivio rear derailleur and an 8-speed cassette. My Alivio derailleur was destroyed in a bike crash, completely bent out of shape, so it had to be replaced.
My local bike shop replaced the Alivio derailleur with an Ultegra long-cage and a narrower chain (to fit through the Ultegra derailleur), but kept the 8-speed cassette and EF50 rapid fire shifter.
The EF50 shifters
The Ultegra long-cage rear derailleur
The 8-speed cassette that is still on the bike, works just fine with the Ultegra derailleur.
The derailleur itself is not indexed. The indexing is in your EF50 shifter, and it has the right amount of pull for that 8-speed cassette. So a 105 or Ultegra rear derailleur should work just fine with an 8-speed cassette as long as you have the right width chain to fit through the derailleur.
If the spacing of the dropouts on your frame is not enough for a 10-speed setup, then you will have to stick with the 8-speed, in which case just replacing the rear derailleur and chain should do it.