As far as the application or actual download speed. If you are on ATT&T the 3G network should be pretty quick as long as you are utilizing the 3G network.
I have an 8310 so its EDGE. I'm also trying to figure out how to delete the default themes. I d/l MaxMem from BB but it scares me a little. I'm trying to free up memory.
One task in this MaxMem program is "Remove Default Themes:Removes all but Dimension Zen"
It tells you to switch to the theme first. There is another theme I like but I can OTA it after I delete stock themes. I'm not very adventurous so if you tell me not to, I won't.
If you delete all of your themes except Dimension Zen you will only be able to download Zen style themes. I wouldn't delete them. . This is a great place to go to free up memory: