My recollection of reading about issues with NOX when the restrictions started (ie. stories in R & T, C & D etc about problems makers were facing) is that it's not the volume of the chamber, rather the area of the surfaces in the chamber that caused the difficulties.
Hence the short stroke engine had fewer of these problems. But the chamber in a side valve is, of course, blessed with a very large surface area.
I'll be interested to know how this turns out...
