lengthen wires
 

in my quest to keep my wires as short as possible, I may have taken too much off my battery wires. Two questions,

Is there any problem if I extend the wires by soldering some more wire onto the existing wires? I can't imagine there's a problem.

Is there any difference really (resistance wise) in just making a deans extender, plugged into the existing deans?

It shouldn't be much of an issue. The "right way" to do it is to replace the battery wire, but I don't think you will notice a difference if you can solder halfway decent. add a length of wire directly to the short battery wire. The controllers like the wire as short as possible to minimize voltage spikes.

resistance
 

any connector vs direct soldering adds resistance. How much is debatable but you probably wanna solder wire to wire if possible - just to be safe...