I am not going to answer the original question exactly as you asked it since I have never used the tarp method but I will give you my experiences and opinion.
I first started out with a 3 bag exmark setup and a separate mower with mulch kit installed. I first would mulch the leaves and then go over them with the 3 bagger. By mulching the leaves 1st you would not have to empty the bags near as much which is the time killer. Then I would empty the debris on the trailer which meant I either had to go and empty the trailer or take a 2nd truck/trailer for the debris.
My present system is a dump bed with a debris loader and a hustler with a BacVac dump collection system. Now I just pick up with the BacVac and dump the leaves near the loader and then suck them up after I am through. WAY more efficient. Occasionally I will mulch the leaves before vacuuming them up if the build up is real heavy. This method has saves tremendous time and you do not need 2 trucks. One person can work alone and make good time if needed.
It sound like some of the yards you are doing on a one-time wait till they all fall deal. I have gotten away from these and pretty much will agree to do these on a spaced out schedule according to how fast the leaves are falling. The reason being when the leaves are allowed to build up too long it causes jams, broken belts and etc. Point being if I am doing a clean up for $200 and I break a $60 belt there goes profit plus down time and frustration. I might do one timers if it is a yard that does not have a lot of trees and the build up will not be excessive.