Yes, thermal updraughts will help your climb performance and retard your descent. In an updraught the air you're flying though is ascending. So climbing in an updraught is like walking up a moving up-escalator. To an observer you will ascend very quickly.
Is it fair to say that during dry, warm conditions, thermal updrafts assist the aircraft in the climb and retard the descent on average?
No, not really. Wet conditions do not necessarily mean no thermal updraughts. Thermals are triggered by thermal energy: surfaces get heated by the sun and cause uneven heating of air (e.g. the air over a concrete parking lot). The increasing temperature decreases the density of the air and this bubble of warmer air will start to "float" upwards through the relatively cooler, denser air until it reaches a height where it is the same temperature (and density) as the air around it. That's your updraught.
With this suggesting the opposite also being true that during wet conditions, the lack of thermal updrafts, the aircraft is less efficient?
Yes, but not for wet/dry conditions since humidity really doesn't have an appreciable effect.
If this is true, are there calculations available for pre-determining aircraft performance and fuel consumption rate in different weather conditions, in particular wet or dry?