I'm not an expert on PHP, but there were a few things such as closure which became available in 5.3 that makes programming much easier, so some apps simply won't run on 5.2. But then there are older apps which rely on some features that were deprecated or dropped from 5.2 in 5.3.
As for FastCGI, I think it depends on the nature of your site and the load patterns per visit. I found that FastCGI actually made my site slower overall. If I understand it correctly, it's because the vast majority of requests are for a single 1k bit of HTML which is returned through an AJAX request. FastCGI seems to improve site performance when there are a lot of requests at the same time. Actually, I shouldn't say that. I don't really fully understand FastCGI, but I did benchmark regular and FastCGI under ridiculously high loads and found that regular resulted in fewer errors from running out of meory and faster overall load times for my particular situation. I doubt, however, that most sites have a usage pattern which consists of users sending 30-50 AJAX requests for every 'normal' page load.