The enhancement on the Gallant Coronet allows you to cover magical spells and attacks instead of just physical hits. Essentially this makes Cover more like a true Cover effect blocking everything instead of most things.
The enhancement on the Valor Surcoat causes 20% of the damage you take from a covered attack to be converted to MP. You still take normal damage, you just get bonus MP everytime you cover. So, if you got hit for about 100, you would get something like 20mp back. This doesn't take into account the rounding error that sometimes occurs, and from what I have seen, the game always rounds your returned MP down.
I have both of these items, so I can verify this is correct as of the time posting. Neither of the items seem to encrease the duration of the cover effect longer than the standard 15 seconds. Only meriting can allow that for a maximum of a 35 second effect.
(Edited to correct numbers.)
Edited, Jun 3rd 2008 9:55am by SeraphPDH