Dedicated lines

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.

I agree with Stanwal, you already got the big bang going with dedicated lines, a splice will not kill those great results.

I would twist the pairs together and seal with a Scotchlok and rubberized tape to reduce air transfer to the connection. You might notice a drop in performance when you first do this but after the new Romex is run in a bit performance should be where you were.

Goes without saying, stay with 10 gauge on both sides of the spice, don't let an electrician talk you into dropping the 10 to a 12 at the splice.

Last thing, most city code on electrical requires a splice to be a box. You might use your current boxes for the spice (obviously REMOVE the AC sockets there) and run the new. If you ever want to reverse the arrangement you only need remove the blank cover.

When running dedicated lines, should I be using romex single solid core cabling or a twisted pair of stranded cabling? My electrician has recommended a hospital grade cabling that he is recommending I use for running new dedicated lines.