Starts as part of warmups are more like real starts #90%. The leadin for flying starts are not really like this.
The concept of flying starts is to achieve a higher top speed. You do this by doing a LONGER and more gradual acceleration, so that you reach your max velocity just before the start of the max speed portion. So the comparison is not between 60 meters from blocks and 30m accel + 30m fly: It would be more like 40-45m accel and 30m fly (which is what Lance Brauman described Gay doing in one of his coaching seminars).
Two other points:
(1) For the fly section to work, you have to be able to produce ATP fast enough for your legs to put out force fast enough to cause the CNS to fire more quickly (which is what flying starts are really supposed to be about). This requires speed endurance training in the 8-15 second range, which causes you to recruit more IIb/x fibers (remember these stop from acidosis quickly) as the initial fibers stop. Otherwise, you run out of energy when you get to maxV.
(2) Flys work for middle distance people because they are not used to training in that speed zone. But for real sprinters, 20-30m flys don't work because max velocity is mostly not trainable. Max velocity is terminal acceleration, or the point at which you cannot put out enough force quickly enough to continue accelerating. So modern sprint coaches primarily emphasize accelerations in the 30-60m range and speed endurance/overdistance in the 80-300m range.
The flys I do as a sprinter these days are 60 meter fly-ins and they are really speed endurance sessions oriented to improving closing speed over 100-200, done this way:
20m drive from 3 point
20m transition stage (up before starting the flying phase)
50-60m at max velocity.
60 meter flys are fairly common in 400m programs these days.