I thought about going with 20"s at one point. I even considered those exact TRD/Enkei Deep Six wheels. They are very nice wheels.
After considering the following, I changed my mind and went with lightweight Volks instead.
1. Performance: Each 20x8" Enkei wheel weighs roughly 36 lbs. That is an increase of 14 lbs over ONE stock wheel, (22 lbs). Total increase in rotational weight from the four wheels is 4 x 14 = 56 lbs. Each pound of unsprung, (rotational), weight is approximately equal to 8 lbs of sprung weight. 8 x 56 = 448 lbs. In other words, 20" wheels have roughly the same EFFECT on performance as throwing 448 lbs of crap in the trunk. Let me ask you this... How does your truck perform when you take three of your 150 lb friends around town? I haven't even calculated the increase from the tires. You can do that yourself. Avg weight of 275/45/20 = 40 lbs as compared with 265/70/16 avg weight of 37 lbs. Source? Check tirerack and search specs for these tire sizes. Main point: Larger diameter rims will have a negative effect on your acceleration, braking, and handling. This doesn't even take into consideration the center of mass and the effect on moment of inertia. Picture swinging a ball at the end of a one foot string in full rotations. Extend the string to two feet. Which one of these is easier to rotate at the same rpm? Can you accelerate them to the same degree?
2. Considering the above, heavier, larger diameter wheels decrease your mpg's. Where do you think the price of oil is heading?
3. It increases stress on wheel bearings.
4. It increases wear on brake pads and rotors. Not sure how your rotors hold up, but traditionally the 3rd gen 4runner rotors are notorious for warping.
Conclusion: Basically you take the overweight, underpowered 4runner that already gets crap for gas mileage and you compound the underlying problems. Is all of this worth it for the improvement in looks - which is purely based on opinion? Some folks understand these complications and are fortunate enoough to have the cash and/or resources available to maintain their vehicles properly. If that is you my friend... all the more power to you!
Just my two cents worth.