Will installing Lion on a MacBook 5,2 improve performance?
I was going to add only a comment, but it was too long. The truth is that Speed/Performance is almost always relative when the difference is not too big. The question is, would Lion be a better option for you? I certainly find some of the Lion "new features" almost a must-have for my daily workflow. The overall OS improvement makes it better for me.

The truth is, that the first releases have proven to be more disk intensive/CPU intensive in a few tasks, presumably because there are more services (autosave, resume, etc.) running in the background. This alone may be the culprit of most of its "performance" problems. It has slightly improved, but I've seen very fast Snow Leopard installs. To be fair, SL have had a few point releases already, and Lion only 2, so one can assume that it will get faster over time as things are optimized (at least this is what has been happening with Apple for the past eight years). 

All in all, evaluate wether you find Lion a good addition to your OS rather than the raw performance. Some people really like the SL "Spaces", Dashboard, Exposé, etc. on the other hand, Launchbar, Mission Control, AutoSave, etc. are features that some people hate, and most of the time, there's no way to revert to the "previous" behavior. 

In the end, you get used to it, but it takes some time.