hmm If they are very wide open maps, they sound like they are, then that can definitely be a choke point. You can always try lowering the screen resolution and other settings to see if it helps.
If you want to delete all the things you've downloaded from servers, go in your home folder and go to Library/Application Support/Unreal Tournament 2004/ and move the folder called "Cache" out. This is where all the downloads from servers are saved.
But remember, you'll have to download everything all over unless you open the Cache.ini, and only delete out the files you want gone.
And again, it definitely isn't a corruption issue. They're just demanding maps for that Powerbook.
Edit: Just saw what Redeemer Sniper Heaven is, that map could definitely be choking the Powerbook in terms of how open it is.
Edit 2: I downloaded this Redeemer Sniper Heaven:
http://nalicity.beyondunreal.com/map_hub.php?mid=8400
It is definitely the map's fault. I was getting ~20 FPS outside the huts. This is on a MBP 2.2 with 8600gt. Normally I get 100+ fps. This map, and probably the others are just very poorly optimized.