I don't think the fee should be destroyed. It would be a much better idea to pay it to Bitcoin miners or XCP holders. No? Maybe I don't understand it well enough.
But yeah 5 xcp seems a bit much.
Destroying the fee
is paying it to XCP holders.
Destroying the fee reduces unit availability, which isn't necessarily an issue if the destruction process will last longer than the system's usefulness. However, I think a better option would be to simply lock the fee (unless called back). Creating an uncallable asset would essentially be burning.