While it would probably be slightly better to run new ones I don't think you will experence any drop if the splice is done properly. If voltage dropped at every splice it wouldn't get very far. I would go with the splice myself unless I was going to worry about it.
Dedicated lines
A few years back, I had an electrician install 3 dedicated 10 ga runs for my audio equipment and the results were really great. I now need to move my equipment further down the wall they are located on and need to move the 3 dedicated outlets. Is it imperitive that I rerun all 3 lines to the new location or can I splice on to the exsisting wires to relocate the sockets. What kind of voltage drop will I see and will this be a bad situation for my equipment?