The concern would be the front brakes since they do the most work, and I would say that - Yes - the stock front disk brakes are sufficient - they have decent stopping power - BUT - since the 4Runner is a heavy truck and with larger tires, the brake pads will run hotter then an upgraded set would, will wear faster then an upgrades set would, the rotors will get hotter then an upgraded set would, and they can warp sooner the normal.
So my opinion is that yes, they are OK but it doesnt take much to get some fade going and the pads are always working like 80% just to slow ya down for a red light on a flat road.
Their are some options such as higher temp brake pads and drilled rotors (which both I highly recommend). I would love to tell ya that you can upgrade to the Landcruiser rotors but actually since your truck is already an IFS its got the larger calipers and vented rotors on board. So your brakes are much better than any thing that was available on the solid axle setups (which were single disk/non-vented). I am not sure about V6 brakes if they are stronger or not, but I think you can use a bigger master cylinder but then your pedal pressure will be greater unless you can find a larger booster (I dont think one exists).
Your brakes are good but you know OEM tires were max. 33" so ........ Marlin's Tundra's brakes ALWAYS warp and go bad. Toyota didnt get the brakes right with that truck. They've since made larger brakes now-a-days for the new models, but in my opinion Toyota could have placed bigger brakes on their trucks.