Detachable (high speed) chair lifts usually do not have a higher capacity than their fixed grip counterparts (aka... there aren't any more people on the hill). The difference in speed is compensated for by wider chair spacing. Detachable lifts must have a wider chair spacing so that the chairs do not bunch up in the terminals during loading while the chairs are moving slowly.
The industry standard capacity for a quad is 2400pph (people per hour), regardless of whether it is detachable or fixed grip. The length of ride is different, but the number of people being put on the mountain is exactly the same. Detachable quads do allow for a higher capacity under special conditions, but it is rare to see a detach with a capacity higher than 2400pph.
It's all one big mathematical equation, in reality. Consider the following:
C= capacity in pph (people per hour)
P = passengers per carrier
T = loading interval in seconds (time between each chair passing the load board)
R = rope speed in fpm (feet per minute)
S = carrier spacing in feet
C = (60 * P * R) / S
T = (60 * S) / R ---or--- T = 3600 / (C / P)
Let's look at a hypothetical situation. Lift "A" is a fixed grip quad running at the maximum legal speed of 400fpm. Lift "B" is a detachable quad with a maximum design speed of 1200fpm. Both lifts have an industry-standard comfortable loading interval of 6 seconds.
First, we need to determine the chair spacing based on T = (60 * S) / R
6sec = (60 * S) / 400fpm
2400 = 60 * S
S = 40ft
Now, we can plug in our chair spacing to determine our capacity based on C = (60 * P * R) / S
C = (60sec * 4people * 400fpm) / 40ft
C = 2400pph
We'll start by calculating chair spacing as we did with Lift "A"
6sec = (60 * S) / 1200fpm
7200 = 60 * S
S = 120ft
Again, we move on to capactity
C = (60sec * 4people * 1200fpm) / 120ft
C = 2400pph
That's theoretical, or, design capacity. Actual capacity is a whole 'nuther ballgame (based on loading efficiency, stops, slows, percentage of load, etc.).