If the titles you installed are signed (aka you can run them directly without launching anything from the browser before) they should still work if you updated. But there is no benefit updating so why would you ?Sounds Good!
I don't know how to remove the update folder on the Wii-U as someone else mentioned. I haven't even got a custom firmware installed as it looked too difficult for me. So this LocalDNS tool is really handy for blocking Nintendo System updates.
I am using Wup Installer and a USB drive connected to the Wii-U etc. Not sure if I can update to 5.5.2 but its still on 5.5.1E currently.
Thanks