I found a way to change the playtime for the gen_intro plug-in while waiting for your addition in the next build.
The configuration data for this plug-in is saved in the "[WinampIntroPlay]" section in the plugins.ini file. The playtime is not included for some unknown reason (so it defaults to 10 seconds), but it can be manually added by inserting a "Time=" variable. I have re-enabled the Settings Cache and noticed this by reviewing the Settings Cache tab in the Preferences - Advanced - Diagnostics options.
The advanced diagnostics is a great resource, I'm very glad you added it.