It seems spring rate out back would be the only thing reducing payload on this model. Seems some airbags or stronger springs would fix that and you would still have the lockers and other goodies.
Springs are one aspect of GVWR rating. Obviously the frame, axle, brakes are rated up to 1400 on some models so there's maybe 300 lbs of payload left on the table due to the ZR2 suspension that you might recover. Still, I wouldn't assume you can perform miracles with just an add-a-leaf.
I'm disappointed in the ratings that manufacturers need to put on trucks. I look at my old 1991 Toyota, which had a GVWR of 5420 lbs and curb weight of 3425 lbs stock. So cargo was 1995 lbs. Mine was roughly 4400 lbs with driver (me), full tank, 'Nest, tube rear bumper, normal tools & recovery gear and ARB w/ winch. So I had quite a bit of headroom (and more so since I ran OME heavy rear springs, GVWR + 400 lbs was the rating) for a passenger, camping gear, water, fridge. The limiting factor was fully laden the engine was just suffering, but it was safe.
Contrast that with my 2008 has almost the same GVWR (5350 lbs) but has added a lot of bulk since the curb weight is 3990 and given cargo is 1360 lbs. I haven't weighed mine but I've added a basic shell, sliders, an ARB is going on soon (no winch yet), extra battery. I'll certainly be right at the GVWR when the truck is closer to done, if not over. It's part of the reason I haven't put on the WilderNest yet. That's 200 lbs heavier than the shell I have now.
Something I'm not sure many think about is GVWR is more than just how much the truck squats. Anti lock brake performance, traction control (if you have it), air bag timing are based on design parameters and testing. If you leave that envelope I start to wonder what are the ramifications. I got GVWR rated OME springs because I'm still mulling over exceeding the GVWR and if that's safe. I went with an ARB bumper because it's crash tested and is said to not affect the air bags (IOW, their timing remains the same). So it doesn't make sense to then just ignore other aspects of vehicle capacity because it's inconvenient.
It's also something to mention that even being safely within the GVWR of the truck the weight took a toll, body seams starting to open and stuff. But it took 280K miles for it to start happening. Even staying within the ratings on newer trucks I doubt will prevent them from wearing out faster. I look at how my body twists and just wonder.
But OTOH, I think the brakes and axles on this Tacoma have some margin so adding some heft (boxing in the rear section, adding reinforcement through the midsection) to the frame might mean the safe GVW is actually somewhat higher than the one Toyota put on it. I'm pretty certain the reason for the GVWR is that they use a 'compliant' frame for people who buy trucks but don't really want a truck. The Hilux overseas still have around 1800 payload ratings, so I think if Toyota was to put a similar frame as the Hilux and 79-95 trucks (e.g. fully boxed) that a Tacoma GVWR would be more like 5800 lbs. Problem is I'm not going to stamp a letter with that statement so I'm stuck with a 5350 door jamb sticker regardless.
