The above commands speed up DCC send only, you cannot speed up DCC Receive.
You do realise mIRC isn't a 'downloading' program, right? It's a chat program, meant for chatting. We won't help with illegal file trading on this forum (including the downloading of mp3s, movies, software and other warez), incase that is what you're doing.
Otherwise, most people wouldn't be 'downloading' large files regularly anyway for it to become much of an issue - if it is an issue for you however, your only option is to upgrade to cable/DSL. If you can't do that due to your location then you're stuck (like me!)