Respectfully, I'll have to disagree. Let me preface this by saying that i personally prefer a V8 for my cars - I prefer simple horsepower - but I've been doing the turbo 4 thing long enough to know that even 100 ft/lbs per cylinder is child's play for most common 4 cylinder engines these days.
Let's take your example of stress limits though, because it does bring up an interesting view point. Turbo engines can easily handle, and make, lots of low end torque. Let's say you have the 2.3L turbo and 3.5L NA v6 both making 280 ft/lbs.
The 2.3L can make that torque probably as early as 1500 to 1800 rpms and hold it out probably as far as 4500 or so.
The 3.5 might not even make that much torque until 4500+ anyways.
So, if I were just chugging along in my car, using the engine's torque to move me through the air, rolling resistance, carry the weight, etc... which engine is actually having to work harder at a cruising RPM of say 2000 or so?
Respectfully disagree to which point? You think you can put a turbo hayabusa engine in a UPS truck? LOL
To answer your question it probably only takes 70hp or so to keep an average car cruising on level ground. Under this condition the 4 pot will operate at a higher throttle position, have less pumping losses and thus consume slightly less fuel.
The problem comes in during transient loads. Going uphill, hard acceleration, headwind, pulling a trailer the 4 pot will need boost to compensate. At only 7psi, thats equivalent to the engine pulling 50% more mass of air than it would under NA operation. The cylinder pressure will be much higher, exhaust temps will increase and the engine will be under a higher stressed condition. You can build the short block components to take this stress but like i said, there are practical limits. This is why you don't see the ecoboost 3.5L used in superduty even though it has greater low end torque than a dinosaur 2v NA V8.