I'm pretty certain that increasing load carrying capacity by upgrading the spring/shock is NOT the same as increasing GVWR. That number is not regulatory-body (NHTSA in the USA) enforced, but rather the weight at which the manufacturer typically runs all durability tests. Sure, if you do, the results will likely not be catastrophic, but for a vehicle that is operated in extreme conditions in remote areas , I'd certainly refrain from exceeding GVW personally. I know that the US OEMs use 90% of GVWR to determine loading on the various components, and that is ultimately the loading at which they will have the longest life.
Let me try to explain it a little better...
Yes, most people understand GVWR as what the manufacturer (Toyota in this case) has declared as maximum operating weight of this particular unmodified vehicle. To comply federal law, you should not exceed the weight on that data plate/placard.
There is what's called "intermediate or 2nd stage or Final manufacturer" and they take a vehicle, engineer, modify it and slap on a new data plate that has their revised GVWR/GVM.
The plate looks like (in Australia):
All "WA SUSPENSIONS PTY LTD" did was change the suspension and the new GVWR/GVM becomes 3300kg. There is a demand for this in Australia and that is why many companies offer it. I bet you if our State Highway Patrols required weigh-ins for light trucks/SUV's, there would be big business for it in the US too.
But there are many 2nd stage manufacturers in the US, most notably with RV's and commercial trucks, that will modify a vehicle and then issue it a new data plate with a revised GVWR. Have you ever seen a Toyota Motorhome? Yes, looks overloaded and probably is once you put 2 passengers in there.
1992 Toyota "1-ton" GVWR 5600 lbs
1992 Toyota Winnebago, outfitted with different rear axle, hopefully brakes and what not. GVWR 6000 lbs
You may or may not trust the GVWR the 2nd stage manufacturer/Engineer's rating. But for FMVSS compliance and ADR compliance, the official GVWR can indeed be changed from the original manufacturer. If you don't need the compliance but you perform the same exact modifications, your GVW capacity will go up.
If you want more info, just search "FMVSS multi-stage manufacturer"
My work truck has one of these stickers on it.