When one is dealing with a five turn primary and makes fifty different transformers, all classed over time for their operational characteristics in the same front end, one eventually compiles huge data sets of said characteristics.
A single, solid 18 ga primary was far less efficient than the final litz wire I ended up using. As I configured each ga and strand choice, I noted the gains and arrived at the optimal ga for the frequency I was running at. It matches the ga in a table mentioned by a respondent in this thread, though I was at a slightly larger ga. Imagine that. I probably could have made it run even better if I had continued into testing a few yet smaller strand choices.
Regardless, the unit went from dropping out at just below 6 volts, which would not meet the spec, to running all the way down to below 3 volts. The designed source is a 9 volt battery.
It also made the choke on the driver FET operate far better. It was 5 turns on a core not much bigger than a quarter inch in diameter. It took testing of several core types and sizes as well as turns count and wire choices.
Yes, optimally, A Litz configuration has to have the strands arranged so that they do not remain in the center of the bundle, BUT the difference is not that great, especially on miniature transformers meant for low power switchers and the like. The fact that a solid core is no longer being used means the effect WILL occur, even if a given strand never makes it out to the edge of the bundle.