At the Kippford weekend I was treated to a few hours sailing with Geoff on his Split Rig Westerly Berwick, here are a few observations (for what they are worth)

She sailed very well, very controllable and responsive. It was quite gusty and I found it easy to pinch up when pressed.  No instruments, but we appeared to be making very good up wind progress.

It was the first sail Geoff had made, and it is a very complicated beast - so well done.  That said there were diagonal creases in both main and front panels.  Adjusting lines did get rid of most of the main ones, though made the front worse.  I suspect it may need some re-cutting to sort it out, perhaps the leach of the front panels was a bit tight?  All said, the creases may be more appearance than performance robbing.

Looking at the set of the sail and how the air may be flowing, it looked more like (true to it's name) a single sail with a split - the air flowing across the lee side, jumping the mast gap.  So perhaps not really working like a Bermudan  jib and main, with the all important slot effect between the two. 

I have also sailed on Paul's Junk Aero Rig, that did appear to be closer to the Bermudan main & jib.   Of the two I suspect the Aero easier to make and get a clean looking result, the sails being flat-cut.  That said, both great rigs.