It would be none of them all. The functions WordPress currently is using are deprecated in the latest version of PHP. This will not effect WordPress for years but WordPress does need to fix it some day.
That is the reason for having this plugin. So we can test things and bring it faster to developers who don’t use WordPress bug tracker that much.
In your case you probably don’t want to use it on a live blog since things can break because some plugins do stupid things.
But understand that if there are functions that are not working in wordpress (because are obsoltes) and this plugin tries to solve that problem, you should notice some improvement when using it or am I wrong?
On speed it maybe be a little bit slower but when using an object cache then you wouldn’t see any speed changes.
The changes are made on code level. MySQLi and PDO are OOP. It’s the future that has been set by the guys of PHP. WordPress will still work perfect till PHP removes mysql_* functions. Maybe PHP 5.6 but it can be later.
It does affect, if not WordPress for end users yet, so at least WordPress for developers when running on PHP 5.5, because WordPress then may emit E_DEPRECATED warnings.
I make tailored, private plugins and wants to use the latest in OOP and other enhancements in the latest PHP release. To me, this plugin already enhances WordPress and I have started enjoying it this first day, after some basic testing.
The plugin description could explicityl say it’s made both as a tool both for developers in general, using PHP 5.5, and, as stated, for the development of the specific enhancement in a WordPress future release, through trac.
