I did this on a Volkl zenit and it really floats well.
I have to say it did loose some stiffness though, probably because it had carbon stringers tip to tail in the exact spot where you cut out for the swallowtail.
It works fine in powder, but has lost some groomer performance without a doubt. You only notice it when on hard not so smooth surfaces.
Probably depends on the structure of board and where the stiffness comes from. For many boards this will not be an issue.
Also a proper powder board will have a smoothly rockered nose and taper, these features will make it better than a DIY swallowtail IMO. Still its a great fun way to breathe life into an old stick and get a pretty good powder deck.