Global Mapper v25.0

Map Catalog for large raster pyramid

Can anyone recommend or share their experience creating map catalogs for large raster collections in a pyramid configuration?

We have a moderately large raster dataset (9000 km2 @ 0.15m pixel) which was supplied in 1km ecw tiles.
We have used gdal_retile to create jpeg-in-geotiff pyramids for geoserver, as this supplies a number of client programs (including Global Mapper).  However, it would be great to access these pyramids directly from Global Mapper, for example using a map catalog.

Can anyone advise if they've loaded large image pyramids directly as map catalogs before, and how they structured the catalogs?

My current issues/thoughts are:
1) At ~100k files, xml length and map catalog load time is significant
2) Ensuring that the correct file loads at the right time, ie image scale 25%-100% of screen size would work, but then final layer drops out.
3) I'm considering splitting the map catalogs either horizontally or vertically, ie: multiple, or one catalog per resolution band.  Both of these approaches have their own trade-offs.

I'd be grateful for any words of advice.