Several points.
Firstly, I'm not talking about Hooper (or any shorter, lighter jumper) being used instead of the locks, but as a fourth main target, in addition to the locks and the six / eight, thus giving us four primary options. This isn't a new idea. Hooper has been used in this role at the Waratahs.
Secondly, faster into the air doesn't just include lift time but also time across the ground in the movement phase of the lineout. A small fast man can change position more quickly than a heavy one. This increases the number of plays available, and thus makes the lineout harder to defend.
Thirdly, on the narrow point of lift time, let's compare Hooper (101kg and 1.8m) with Francois Louw (a six who is 113kg and 1.9m) and Kane Douglas (a lock who is 123kg and 2m). Say target hand height is 4m. Say resting raised hand height is player height * 1.25. Gravitational potential energy is U=-GMm/r. For Hooper, U=1732 joules. For Louw, U=1799 joules. For Douglas, U=1808 joules. Thus (for those variables) more energy is required to lift Douglas to 4m than Hooper, and thus the same force will lift Hooper to 4m faster than Douglas.
Obviously a string-bean lock would be a different matter to Douglas. A 110kg 2m lock needs only 1617 joules for the same result, and thus would be fastest of all in the lift phase.
Finally, we don't pick tall locks because they're faster into the air, but because their upper limit is higher. This is to avoid the problem of the tallest man, in which one team can get one man higher than the other team's upper limit by more than the reliable variance of the hooker's throw, and can thus win every lineout simply by throwing it higher than any of them can be lifted.