I've never seen a wireless card transfer at 54Mbps, much less hold a connection with that rating for long (A router company got sued not long ago for printing this spec on it's router box, and didn't live up to it). The reality of it is, your ISP is only allowing so much bandwidth to you, so the only place you should see a difference would be on your own network transferring files over the network. Most ISP are only allowing 8Mbps download and 512Kbps upload speeds. I know Comcast has 8Mbps, and Bell South has 6Mbps (in and around my area) download speeds- if you're willing to pay the xtra $. The 54Mbps- maybe in the future when all networks are made of fiber to our homes
No, securing your network shouldn't slow it down. The only way a program is going to slow down your bandwith is if it is downloading massive amounts of info at the same time you're on the net doing something else.