The DB601 was intended to operate from ground level all the way up to 30,000 feet. A broad torque curve provides a significant advantage in that situation. More then enough to offset the disadvantage of power loss from coupling slippage.
I think we have a bit of confusion here. The supercharger drive has nothing to do with the "torque" curve of the engine. If you are referring to the power-output in relation to altitude curves or plots that is something different.
You also have to realize what the different drives can and cannot do. While the DB 601 may have operated "from ground level all the way up to 30,000 feet" the early versions had a full throttle height of between 4-5000 meters depending on the model. power fell at exactly the same rate from that height up to 30,000ft or beyond as if the engine had a plain gear drive for the supercharger.
It is below full throttle height that the fluid coupling provides an advantage. While it takes a few more HP to run than a straight gear it does allow the impeller of the supercharger to turn slower at the lower altitudes and so use less total hp, this leaves more power for the propeller and it heats the intake air less. This is what the low gear in a two speed supercharger does also.
The Fluid coupling smooths out the peaks and valleys or the zig-zag of the 2 speed drive but only below the full throttle height.
Consider the Merlin III and X. The Merlin III had a full throttle height of 16,250ft (4900meters). Power fell off above this height and below. The throttle had to be progressively closed as the airplane lost height to stay within safe operating limits. The supercharger continued to spin a the same speed and so used just about the same amount of power. By the time the plane got to sea level the engine was limited to 880hp instead of the full throttle 1030hp at 16,250ft. The supercharger used an 8.58 drive gear.
The Merlin X used the same supercharger but had the two speed drive with 6.39 and 8.75 gears. Since the power to turn the impeller goes up with the square of the speed we can figure that in low gear the supercharger was taking about 53% of the power it needed in high gear. Full throttle heights were 17,750ft but at a slightly lower power than the Merlin III and 5,250 ft in low gear. The 1130 Hp at that height was a significant advancement over the Merlin III as was the 1075hp for take off. Almost 200 more HP with only a change of supercharger gear.
Fitting a fluid drive instead of the 2 speed drive, if the range of speeds of the fluid drive were limited to between 6.39 and 8.75, would show no increase in power over 17,750 ft and no increase in power below 5,250ft. It would eliminate the dip between 5,250 ft and 17,750ft. and show the most advantage around 10-12,000ft.
It also makes life simpler for the pilot because the early 2 speed drives usually needed to be manually shifted. Later ones were automatically controlled.
The DB drive did exhibit a slightly wider range of ratios than the above. From about 7. something to a max of 10.39? with different intakes, impellers and diffusers direct comparisons of impeller rpm don't mean much. but a slightly wider speed range would be an advantage.