Whatever figures people are quoting for their fuel economy aren't important. What are important are the inherent characteristics of both the V6 and V8 engines. The V8 produces around 13% more horsepower (max), 15% more torque (max), has two additional cylinders and weighs ~200 lbs more than the V6 engine (I'm using 2006 SE edition). There is no way - repeat, NO WAY - that the V8 engine will match or even slightly trail the V6 in terms of fuel economy. Think about it - if it was so easy to improve performance while barely affecting the fuel economy, why would engineers even bother with a V6? If the V6 gets 19 MPG and the V8 gets 18 MPG, who in the world would even consider a V6?
Forget what the EPA ratings say - they're crap, and we've known that for years. The V8 is going to use significantly more fuel because it is designed to; We're feeding more pistons to produce more HP and Torque. I think this discussion is akin to discussing the recoil between a .38-cal and a .40-cal; the .40-cal bullet is heavier and produces greater muzzle energy. Therefore, the recoil will be worse. No matter how we try to sugar-coat things, the physics behind the reaction cannot be negotiated.
My previous ride was an E39 M5, which had a marvelous V8 394HP/368FT-LBS engine that leapt like a lion when I stabbed on the throttle. Which brings me to another point - V8 driving habits. Nobody who has a V8 tiptoes around town - we USE that torque because it puts a smile on our face. I would call it a "common sense" theory that if we took 10 guys and let them drive V6 4runners half the day, and then V8 4runners the other half of the day, the data would tell us that people absolutely had heavier feet in the V8.
Don't get me wrong, the V8 is the preferable engine for folks who 1) want a muscle-car feel in an SUV and/or 2) tow regularly. That's fine, but don't be fooled into thinking that the fuel consumption between the two models will be negligible. The V8 has about 15% better HP and Torque, and I'd be willing to bet that on average it gets about 15% worse fuel economy, too. Add on the "performance" penalty for lead feet, and I'd set a safe estimate that the V8 gets about 20% lower fuel economy than the V6.