I do stress analysis and certification of aircraft structure. Not going to argue MGW (max gross wt.) but a few comments.
Gust load condition will produce higher G loads when operating an aircraft at *lighter weight*, for a given speed. The lighter AC weight the lower maneuvering speed (Va or Vo). Refresher, big fan of Rod Machado.
1800 lbs RV-6 is totally arbatray set by builder based on operational needs. Experimental airplane, end of story. [Question: Does every builder go through and evaluate all the performance and speed limits maticuliously during phase 1 at higher 1800 lbs gross weight?]
We don't know how Van designed/sized structure, set original MGW, determined limiting critical load cases and structure. Van is a degreed Engineer and sure he did "classical stress analysis" to size the structure (and was conservative). Van I also assume used certified aircraft 1.5 factor of safety [limit/design load x 1.5 = ultimate load] to size his structure.
I have not reviewed Van's stress analysis. From +40 years and thousands of RV's flown safely we can assume Van did a good job (including the few in-flight breakups which had causes other than deficits in design). With that said you can break an RV if you do something stupid.
RV-6 has an aerobatic [1375 lb
] MGW at +6G. Normal category planes are 3.8G. You can fly your RV-6 at 1800 lbs or [30%
] higher than published aerobatic Wt. and wings will not fold up, if you fly your RV like a normal category plane, straight & level. Regardless increase of MGW from 1600 to 1800 a 1.125 factor increases stress and reduces your margins in the air or on ground. [edit
Exceeding MGW on GA planes is often an issue with low performance GA planes because they will have marginal performance. RV's have an abundance of performance. However at 1800 lb RV on hot day, high DA is not going to perform like a light RV. Also as mentioned aft CG makes it unstable as well.
There have been RV LOC (loss of control, stall) accidents where over gross Wt. and CG was a factor. In fact LOC is one of the biggest causes of accidents. Higher gross and stall speed is not helping.
Bottom line higher gross weight lowers performance and increases stress on the aircraft. Is it unsafe or critical?That's up to the builder and PIC? It's an experimental aircraft. Obviously at operations at 1800 lbs max gross weight one should limit the G loading on the aircraft and land smooth, while avoiding max performance T/O and Ldgs.
Last is Joy, I have flown very light RV6 with a wood prop and they was delight. I've flown really heavy RV6's and they lose a lot of the feel and enjoyment of a light RV6.