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Good morning, Rob...
"...the engine revv higher in a lower gear?"
Yep.
"...at 25mph in 3rd gear and at the same speed in 2nd, wouldn't the engine be producing more power in 2nd gear..."
I think you're confusing your engine's wide-open-throttle performance with what's required to maintain a fixed speed, say 50 mph.
In your example with a vehicle at a fixed speed, other than the slight extra losses caused by the higher RPM, the engine will produce the same power. Of course, in some circumstances, an engine is unable to produce the needed power, and hence we downshift from 3rd to 2nd.
Power is the product of torque times speed (rpm). So if you halve the speed, you must double the torque to produce or transmit the same power.
The relationship is: P=TR/5252 where "P"=horsepower, "T"=torque, and "R"=rpm.
The clutch is between the engine and tranny, and obviously transmits (only) torque. If called on to transmit too much torque, it slips.
To drive a car at a fixed speed, say 50 mph, you can run in 2nd gear, and the engine spins fast and produces a certain torque—and whatever it produces is sufficient for 50.
Then, if you shift to 4th gear the engine and clutch speeds drop. But you must still send then same final power to the differential to drive 50. The clutch is now going slower, so must deliver more torque to maintain the same power at the diff.
"...that this engine produces more torque at around 3000rpm than it does at lower revvs..."
Yes, but that's not the issue. Your engine can produce increasing torque at higher speeds AT WIDE-OPEN-THROTTLE. We're not discussing engine performance. We're discussing clutch performance and what the vehicle requires for power at a fixed speed.
"...why would the O/D have an affect on the low gears when it's locked out of everything but 4th??"
You're right, it doesn't. The only comment I made was that the clutch must again deliver higher torque when the driver shifts from 4th to 4/OD, because again the RPM drops—but exactly the same power is required at the differential to maintain vehicle speed. Thus, the clutch (which is before the OD) must transmit higher torque.
4th/OD places the toughest torque requirement on the clutch because the engine is going slowest.
Don Foster (near Cape Cod, MA)
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