Eric Lindbergh's Columbia was flight tested to 4260 lbs.  It had a fuel capacity of 310 gal.  That's about 30 lbs/sq ft. wing loading at takeoff.  I believe the landing lights were removed from the leading edges of the wings to allow for fuel bays to be installed.  Lancair described it as a "stock 300", except for the modifications for additional fuel.  It was expected to land in Paris with about 65 gal. of fuel.  That would put it within normal limits for landing.

 

Lancair press releases describe "extensive heavy testing" that was done before delivery to Erik.  The key here is "testing".  You need to test.

 

I think the structure of the airframe is not the issue with increased weights.  It's the aerodynamics that can jump up and bite you.  Takeoff and landing distances, approach speeds, etc, all need to be tested at various CGs.  It is highly unlikely that a 10% increase in max. gross will introduce any unexpected characteristics.  But the farther you go, the more you have a chance of discovering a "surprise".  Extrapolating outside the test envelope using formulas, or simply guessing, is not wise.

 

Something to keep in mind, certified birds in the normal category are designed to withstand +3.8g to -1.5g.  So every pound you add to your gross weight requires ~4 lbs. of extra structural strength.