Speaker wire selection for inefficient speaker?

That is what the 5db boost is for. To equalize the channels for your seating position..

This is not valid advice, in my opinion.  Length times DC resistance per length unit tells you the series resistance a wire will add. A center channel wire under 6 feet long would not cause measurable voltage drop even at 16 ga.  But this kind of thinking made Noel Lee a very wealthy man!

That said, upgrading the wire to use better materials and construction may improve your listening quality…but not just heavier gauge OFC.  For example, Nordost White Lightning or Purple Flare are light gauge wires, but many reviewers recommend them for clarity, smoothness, and musicality.  Spend a few bucks and upgrade all 3 LCRs.  Other brands may suit you.  Look at Cardas too.

A little perspective.  90+% of the problems are associated with too small a gauge wire. This is especially important for inefficient speakers.  So I always use 10awg wire for speaker wires.  From there you can spend thousands of dollars on the last few percent but the key is heavy gauge.

@alexb76 , its fine to boost the center, that is why the receiver has that feature. If you want to change speaker wire do an upgrade for all of your speakers, Having said that I doubt the wire is whats causing the volume difference. I would just boost the center like you have and leave it.

To answer Kota, I have a receiver with tone control, and an external amp. When I run tone test check, Center is always the lowest and needs +5 dB increase to level out with my left and right channels (B&W 805), hence trying to help it a little but as I assumed and others stated, a wire that short shouldn't matter in terms of resistance. 

Cables will not make a difference. I agree with @fuzztone. You rebalance your surround processor. It will do the job for you. You go through an cycle to set volumes and delay times.

This is a good question. Center channels carry a huge percent of movie content. Make sure the tweeter of your center is the same height from the floor as your L-R channels so they are all on the same plane. Next, use a sound pressure meter and run the test tones from your receiver, is the volume the same in all front channels? Do you have to boost it? If you are using the same cable on your other speakers it shouldn't matter.

your cable is just fine, changing it won't change anything regarding efficiency or how loud it will play - just make sure terminations and connections are sold

No. Turn it up on you AVR.