I'm afraid I am not going to be provider of any answers to this, but hope to expose some thoughts on the concept.
If you follow some of Zenith thoughts on wing tips, they describe in detail of the classic "Horton" or "Stewart" strait top, bottom flat angle method.
The biggest can of worms here is simply being, at the speeds of the XL, do we gain or loose with the added weight of these wing tips vs less drag? (we don't have much to spare in extra weight)
I have yet to see, a single posting of someone flying the XL with NONE then adding any other wing tip. Of course the conditions for testing would have to be identical, and if you need such precise instruments to calculate actual gain/loss, then one would have to wonder if the extra effort was worth it.
All that being said, done right, it could certainly look nicer.. All in the eyes of the beholders though.
cheers
