Speaker Placement Issue/Thoughts

I have a similar issue in my non-symetrical room. My expericnce has shown that it only takes a couple of inches, but I guess that will depend on factors secific to your situation.
As I see it you hae 3 choices:
1. Move the left speaker to the left, thereby moving it farther away from you
2. Move the right speaker to the left, thereby moving it closer to you
3. Move your seat to the right, thereby moving the left speaker farther and the right speaker closer

You could also do a little #1 and a little #2

If you've already tried swapping left/ right inputs to eliminate a mismatch at the source/cabling, try swapping the leads from the amp to speaker- in other words- remove the cable from the left speaker terminals and move them to the right speaker and vice-versa, leaving the connections at the amp end the same.
If your speakers are toed-in at all, try changing the toe-in angle on one speaker only, leaving the distance from the inside-rear corner of the speaker to the back wall the same-in other words:pivot on the inside front spike. See if the image moves. Don't be afraid to try a drastic change to make it obvious.If there's an equipment rack between the speakers check the distance from it's edges to the speaker- this was messing up the center image on my system at one point- making the distance equal took care of it. I put blue painters masking tape on the floor before I make these adjustments so I can always get back to where I was. If all else fails, move the listening chair a bit.

I have lived for years with a similar situation. The balance seems always weighted toward the left side. I tried everything you can try (speaker placement, component replacement, etc.). I have even had my hearing checked and found no difference, right to left. I have now--after years of torture--simply adjusted the balance control (or the volume controls on my Audible Illusions preamp) to establish the true apparent balance and have learned to live calmly with the situation.
It's living comfortably with the skewed balance control that is the hard part.

Lindsey Buckingham talked about this when he released his last solo album...he mentioned how there is no real center image as things naturally shift left in studio recording...which is why i switched to bipolar rear driver speakers...they throw a huge center image...

I had a similar experience only to find out I had a cable out of phase. Check all connections. Good Luck