No matter what you change the padding to, anything over 4 minutes will revert to 4 minutes when you restart the PC or the MCE services (it's somthing coded in MCE).
Your only real options are to choose an advanced recording whenever you select to record something from the guide - that allows you to set hard padding for the particular recording (5, 10, 15, 30, 60 mins), or if you use IceTV as your EPG, their software has an option to set the soft padding (what you're trying to change) to a new default over 4 minutes (e.g. 15 minutes) - there are mixed reports about how well it works though.
Justin