Hmm... yah I guess its pulling map images to memory on first request. On closer inspection I do see a split second blip of disk activity when first calling a map style. The main point though is that it doesn't load map images when zoning.
FYI, MiniMap loads the maps from disk on request even if you haven't zoned there yet or opened the main map
The reason I think the Map file is regened on Zoneing to include maps and styles sent by the server is that MiniMap can't load server sent images befor you've zoned there. Also the Map window closes on zoning. This leads me to believe the map styles for server sent zones simply aren't there to be referenced. If they were, giving MiniMap the right style ref should request the map be sent down from the server.
Its possible all the styles are inserted when you log in and the server's map images are only loaded when you zone there.
In either case, none of that changes due to the Map mods. In fact its possible you'll zone faster since the server obviously uses local styles/images rather than sending down its version.