I too have a SP300V and wanted to have the computer in the next room...but through usb, Rolands states in the setup guide to not use anything over 3 meters (9 feet) long and dont use a USB hub. Its on page 33 in my roland SP300V Setup guide (from 2005, yours may be on a different page).
I wish I could go longer too (like the network cable of the VP), but I just have a 6 foot USB cord hooked into my Rip/Print computer next to it on a desk.
So I think your out of luck there unless you can call your Roland rep for suggestions. Keep us in the loop on what you find out if so, thanks.