I say go for it! I only wish I could be on the plane watching you stroll down the aisle in a gown. The look on the flight attendant's faces would be worth it alone.
On a more serious note, I've flown upper class on Virgin SFO - LHR and they provided a sleeper suit to everyone in first. I'll never forget either, as all 12 of us changed into identical outfits (this was when they were very high quality - I still have mine) and went to sleep. If only we could have had a picture of this.
It makes a huge difference to change into sleeping clothes and change back upon arrival - you feel a lot more refreshed.
Have a blast!
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