Frstnflt,
You actually answered your own question.
As you surmised, installing higher numerical diff gears would give you some extra pull in first gear with your manual transmission. Beyond that, all it's going to mean practically is that in each gear the engine will be turning slightly (depending on how drastic you go) higher rpm. So you might wind up in a situation where you might be doing, say, 70mph in 6th gear at 3,000 rpm, where with your stock gearing you'd be doing the same speed at the same rpm in 5th gear. But so what? The engine doesn't care what gear you're in.
Furthermore, both your fifth and sixth gears, IIRC, are overdrive gears, and thus slightly weaker than the 1:1 or "underdrive" gears. Pulling a load up a hill the transmission is better off in the lower gear. The simple fact is, as was mentioned, these engines produce their highest torque (not to mention horsepower) at absurdly high rpm, and you need to keep the engine up in that region to get the best power.
Also, remember that a higher numerical ring and pinion is weaker than a lower one. It's been argued ad nauseam how much this affects durability and reliability, but the indisputable fact is that a 4.56 r&p is weaker than a 4.11 r&p, and a 4.88 or 5.29 are weaker yet.
If you plan to install taller tires the equation shifts a bit, and re-gearing will compensate somewhat for the added diameter. But you'll still be weakening your diff, and compounding that stress with the added rotational inertia and mass of the larger tires.