You really shouldn't experience speed drops when using wireless. Even at 15 megs, that's only using about 1/4 of the older G standard bandwidth.
Also, a new modem and drop will not mess up your port forwarding. In fact it really doesn't have anything to do with it. What can mess with port forwarding though is if your device has a new IP. Then you'll have to update the port forwarding to the new IP or nothing at all will work like you describe.
20/2 Suddenlink : Current
5/1 CMA : Old
15/2 TWC : Old