Your findings (importance of gapping/constant inductivity even in PP OPT) are very similar/identical to "Lundahl" and "Intactaudio" design philosophy...but, of course, opposite to one another company we can`t name...(or in other words:"The name that should not be spoken..."
Did you see this graph for one R-core OPT? Those kinds of data are rare in the hi-fi industry...
Can we finally get those Cw numbers per PP phase? Graphs, please?

• http://www.icl.co.jp/audio/english/RX40.htm