Originally posted by GeneV I'd agree that a lens would probably make more difference overall than the camera but I'm not so sure that is what he was seeing.
The 55-300 is definitely a bit better, but I haven't noticed that it is superior in the areas of focus lock and WB, mentioned by the other poster. True, the 50-200 needs bright light to really shine (so does the 55-300 IMHO), but for something that compact in a travel kit, it does a nice job.
The 50-200 is a quite interresting lense because it is very small, very light and has a WR version. Despite what the tests say, I continuously see great images taken with it with great colors.
Now one need to know what priorities he has. If the issue is low light performance, the 50-200 is not a good choice. One can replace it with the tamron for a very good price but sure it is quite big/heavy. If that not an option, then in the still big/heavy departement but significantly lighter/smaller, there the 50-135 a really great lense, still with f/2.8.
If one want to be very light, at this range of focal length, I think there no option but to go for primes. Depending of what focal length are the most important DA50, DA70, FA77, DFA100 macro, F135 or FA135 are all quite reasonably small/light and come with fast apperture too. Going with say DA70 + F135 would replace entirely the 50-200. Just crop a bit when the framing doesn't match.