What about the distance that one box might take?>
What about, since everything is on a slab in Texas one uses a lot of PVC ?
The bottom line is going to your boss, not the workers, how American!
As said it has good and bad. The boss knows what labor is going to cost whether it is done in a day or five days, and only has a problem if it is taking too long or if work is not done correctly - but that can be a problem with other labor arrangements as well.
It can motivate workers because the more projects they get done the more they make, but they still need to limit the pace enough that they are not making too many mistakes. They also need to limit the pace enough they don't burn out.
Having correct supplies on site when needed is a must or there will be a lot of conflict as these workers will be complaining they are at work but are not really being paid properly if they can't do what they are being paid to do.
Truck drivers that are paid by the mile are similar in some ways - my father used to do that and ended up really upset with one larger company he once worked for. They generally are only paid for "loaded miles" so when you unload the trip to the next pick up is not paid - companies generally try to get you to a close by next pick up - but when working for a larger trucking company that may mean longer wait times because someone else is closer to some of those potential next pick ups. This gets frustrating because you are miles from home not driving so you are not being paid, but there isn't a whole lot to do either but sit in the truck. Kind of same thing happens if the installers show up to site and there is no material to install.