By what I've observed:

Selling it should bring you around 500k cr.
(I saw a book in the auction house today with a buyout of, like, 5,000,000cr. Though, it was at "very short" and the current bid was only around 520,000cr, so I'll assume it sold for around there, and it'll only really go down as the event goes on.)

That's around 7,500 ce, if you were to convert it to ce now.
(500,000cr/6,600cr per 100ce=75.75 purchases of 100ce)

Now, you can do two things:

Sell the thing. That's enough ce to buy your way from full 3* gear to full 5* gear, with plenty left if you remember to use mist to take 100 off every craft.
(800ce -100mis t+ 400ce - 100mist = 1000ce per 3* crafted to 5*. You'd have enough to craft a hat, armor, shield, and all four weapons, and 500ce left over to buy the weapon slots for those other two weapons)

Or you could keep it. IMAGINE the jealousy of other knights seeing you running around with this crazy hat that two of the rarest two performance boosters in the game in the same place. (+damage high is unique to the hat, and the +MSI gear's main attraction is the +MSI.)

TL;DR: Keep it if you like the game enough to work up to 5* gear the normal way, sell it if that'd take too long. You should probably check the math yourself if you do sell it, though, I'm wrong sometimes.