I guess that you're talking about PHP4 built-in session support, right? If so...
1. I can't say how big performance loss would be when using transparent session ID handling, but I can say that transparent session ID handling in PHP is very stupid... I've seen some web sites where PHP has added session ID even to links that point to JPEG images!? And that's really unnecessary. Also, PHP won't correctly pass session ID to scripts that you call via <FORM> tag (you'll have to add hidden <INPUT> tag with session ID by yourself). So, my suggestion would be to add session ID to all links by yourself (and not rely on transparent session ID handling)...
2. No, there isn't.
3. That depends on server load. You should monitor how many sessions are active and configure garbage collection accordingly.
4. You do know that PHP stores session data on disk and not in database? If you want to store session in database you'll have to write your own session handling functions (for saving and reading session data) and use them to replace built-in functions. Now, that we have cleared that, how many connection MySQL can handle depends on your server (hardware) and of course how MySQL is configured...
