I was pottering about in the loft yesterday, and started looking at a XE head, and started thinking....
at the side of the chamber (taking the inlets at the top, and the exhausts at the bottom), the walls are slightly undercut, and there's a ridge there. The ridge (as I understand it) is there to aid flame travel, but the slight undercutting made me think, especially when reading this article.
if this undercutting is removed (there is enough leeway to do this, it'd leave the head gasket flush against the block and chamber), this would mean that the full force of the combusting gasses would be deflected onto the piston, instead of some energy being lost pushing against the cylinder head, and then turbulance when the piston moves down and the gasses go past a sharp step (the undercut).
I'm tempted to remove it, except that it might be aiding swirl.