Actually, that is false and there is no sure fire way to fix or redo it to not slow down the 360 load times. The 360 is "like" a computer, but it does not operate like one. For this particular instance, you are referring to a computer using RAM, to store recently viewed or used items in operating memory, which take low priority and do not get stored as dedicated memory areas. The xbox 360 dumps all memory and temporary data every so often, especially after matches or when switching maps. The 360 does not have enough available RAM to keep buffered data locked so it can be accessed quickly. This is why even when you play a map like Powerhouse (125mb), then play a smaller one like Swordbase (115mb, then switch back to Powerhouse, it still takes the same amount of time to load it. Even though only 125mb was used to load Powerhouse, this was in it's compiled, compressed form. In reality, a single multiplayer map takes up to as much 1 gig of ram or more to load it uncompressed. Remember, not all the tag data the game needs is inside that one single map, it must also uncompress and load resources from the mainmenu.map file as well as the shared.map file.
That being said, you stated that 50-100 maps was not enough, but now you wanna complain that after having 300 you need a better way to sort it out? Heres an idea genius, WATCH WHAT YOU DOWNLOAD AND MANAGE YOUR MAP SPACE BETTER! No amount of organized UI's will help you, if you keep downloading maps, forget what you have, or have maps that you dont need. Sure it would be nice to browse large collections of maps, but how many of those maps that you had did you "actually" use often? I'm guessing maybe around 20-30 maybe different ones a week. Anything above that, and you would either have to be not working, not going to school, etc... to get that much play time.
The point I am trying to make with wall of text and rambling is this: Due to hardware restrictions there is no likely way to slow down the constant lag your system will expereince once it reaches a certain amount of maps in the list. Map lists are not just simple text that can be loaded. each map is quickly scanned and the system gathers it's name, date, etc.. from the actual map save file.