How much (maximum) torque can a normal 150 lbs. man deliver manually with a hand-ratchet wrench (say 8" ratchet arm and a 1/2" socket) assuming no rust-resistance, direct/easy access etc?
Okay my curiosity was piqued. I am an average 60 something year old guy who turns a wrench for a living. I clamped my old school Beam Type Torque Wrench in a vise. Using a 1/2" square x 3/8 drive drive socket on the torque wrench and an 8" ratchet I hit 70 ft-lbs. This was just a straight steady pull. I did not test to see how long I could hold that number.
Torque is simply the amount of force you apply (lbs) multiplied by the distance from the thing you're rotating (ft), hence the unit "ft-lbs" (foot pounds). If you apply 50 lbs of force 6 inches (.5ft) away from the bolt head, then you're applying 25ft-lb of torque. The same 50lbs applied at the end of a 2ft wrench would give 100ft-lb of torque.
If you can apply upwards force using your leg muscles to supply the force and just hanging on with your arms you can probably put three times your weight on it so 450 pounds on an 8" handle is 300 foot-pounds.
if your're a weightlifter or other athlete probably much more. (you'll probably break the handle and skin your knuckles)
If torque is important use a torque wrench. if doing up wheel nuts stand on the end of the wrench, that will be "enough". (to undo face it the other way and jump on end, that will be more than "enough")
It depends how long the "cheater" is. With a 3 foot cheater pipe over the handle ,it will be a LOT of torque. For my 3/8 drive , 1/2" thinwall conduit is a perfect fit over the handle so I have about a 10" length in my tool box. Large box wrenches ( like 2") in industry only have about a 12" stub handle , the user supplies what ever length pipe he wants for torque.
Thinking more of the underlying question, max torque isn't the only tradeoff between hand and power tools:
The power tool will provide the rated torque for as long as you can supply power (e.g. charged batteries). Can the person? On the other hand if you're working away from power most people would last longer. The power tool's maximum torque is likely to be with a fully charged battery, and will drop off before the rated runtime, and is a hard limit, while you can often find a cheater for a hand tool.
Many light high-torque power tools don't really have a suitable grip to resist that torque, at least not more than briefly, and can be quite harsh on your wrist if used to give maximum torque for long periods, especially single-handed. Heavier impact drivers are better for this for the same grip (conservation of angular momentum) but then you're holding a heavier tool for long periods, which is hard work in some positions.
An advantage of a (typical) impact driver over a ratchet&socket is that the power tool provides closer to pure torque, while a handheld spanner can require a lot more bracing of the workpiece to avoid moving it. That's fine if it's a car or a building, but for something lighter it can be troublesome (e.g. undoing the crank bolts on my bike - I have to do it on the ground rather than the workstand because the down/up force of the hand tool is too much for the stand - I wouldn't use an impact driver there anyway)
When comparing different power tools, runtime and ease of use (ergonomics, battery options) are at least as important. I often do the slow heavy part of the job with hand tools but use my non-impact power driver for the boring spinning of an overly-long bolt.