On my OEM G35x Springs, Rear.
With 225/45 18 Toyo, 18x8 Piaa Super Rozza.

My Rear Fender Gap left vs. right is very uneven. Dealer provided no help
when I asked them about it.

LR: 3fingers~2.25" Gap
RR: 4 Fingers~3" Gap.

Now, I plan to switch only the RR spring to Sedan Sport, this willing drop it 0.5" on the RR only.

Does this sound like a good idea? I know the sprig in Sedan sport Rrea is ~340? & the G35x Spring Rear is ~290. Willl using different spring rate LR vs. RR a problem?

Personally, I think the uneven fender gap, which means uneven ride height would me more of a problem than same ride height at slightly different spring rate. Most of teh time, teh car would see bumps one size at a time.

Help.

Vehicles are designed to be levelish with the drivers weight in the seat and a full tank of gasoline.

3/4" compensation would require 150-180 pounds in drivers seat.

^ that sounds like it would make his problem worse.

That's correct. The driver side is the side that's low,
the fender gap on driver rear is smaller before loading driver. I can only imagine
it get worse after I sit in.

plus, you're talking about 4 springs even if distributed.

would take significant weight to compress 3/4" distributed on the 4
springs.

The center of the drivers seat is left of the vehicle center line but very near the center of gravity [depending on fore aft adjustment].

So 66% of the weight would be on left springs.........measure the distances between the springs [don't confuse track which is distance between tire centers].

Don't confuse the spring free stiffness with the effective installed stiffness.

Measure unloaded then measure drop with known passenger weight you will see WHY the max load is as low as it is ~~500 pounds per inch evenly distributed.

Still, why is my left rear spring sagging with no one in the car?

If I install a shorter spring to the real right only, would it be a good idea?

Examine the unmounted spring free length per FSM, examine the LISTED fender gap [CALLED WHEEL ARCH] in FSM. Check any rubber noise isolation doughnuts.

Not sure you will have much luck, but when a specification is found in FSM the factory should correct.