I did that with a wireless bridge to my router for the ethernet connection. Downloading was fine but uploads to my site were getting corrupted 90% of the time. Took me ages to realise it was the ethernet/bridge that was the problem. I am now back on a USB connection - fortunately with excellent speed.
My TUIT moment was finally giving BT Broadband the big heave-ho and migrating to Zen...took only a couple of days and the migration carried out by Zen was flawless. Kicking myself that I didn't TUIT sooner as Zen far outranks BT in every aspect especially support.
The speed limit is the broadband connection, not the USB (or Ethernet) connection.
USB 1.1 is 12MBPS max., USB 2 is 480 MBPS max. and the broadband speed is only 0.5, 1 or possibly up to 2MBPS.
Etherent may run at many speeds depending on hardware but could be a only 10MBPS
I don't see how having a faster connection between the modem and the PC/mac can make the slightest difference to the internet connection performance unless you have some pretty serious faults in your kit. Regardless of the connection, the actual internet speed is only a small fraction of the PC-modem connection speed.