So, the reason I procured the supposedly dead devil was to find out if it really made a difference. I know that people generally don't age Duvel or beers like it so I know it might not have improved, but does it really go bad? For a bottle conditioned beer with an ABV of 8.5%, I can't imagine much damage would have been done. Let's see then.
The old Duvel pours much like a fresh bottle - slightly hazy yellow-golden colour, but with a foamier, soapier head than usual. Carbonation seemingly unaffected too. There is a tad bit of a difference on the nose however - it's yeasty, malty and fruity and smells quite like champagne. Initially, it tastes like champagne too, before the bready malt qualities, grapefruit and caramel sweetness, and dry alcohol heat at the finish remind you that it's Duvel you're having. The body is medium and carbonation feels light on the tongue. The slightly weaker mouthfeel is probably the biggest loss inflicted by the aging.
It's still a tasty and enjoyable drink - I polished off the whole bottle in a night - and at the reduced price it's much better value than buying a fresh bottle. I do concede that the aroma and taste is a bit more complex in a fresh bottle, and the mouthfeel a bit better, but the positives really outweigh the negatives here.