At my shop, there are only two of us who install, myself as hourly and my manager boss-man on salary. In my experience, the best way to loose long-time workers and completely alienate them is to change how they are paid. I can guarantee you (because I have worked many jobs from corporate drone to mom and pop shop and have seen first hand what a drastic policy change does) if you switch the pay style from hourly to per job, you are going to be faced with a lot of resistance and layoffs. One company I worked for intentionally did something similar - they had a stagnant workforce that had been there for 10+ years, new management felt they were beyond retraining so they instead implemented new extreme policies (intended to improve productivity) knowing full well that there would be a massive upheaval between people quitting and being fired which would allow them to hire new blood under the new policies.
If you value your workers, I would instead focus on training and pay raises. I would also come to terms with the fact that in this line of work, a plateau is inevitable. Unless you want your workers cutting corners to meet unrealistic time frames, you need to accept that a job well done will take a fairly fixed amount of time.
Before I started, my company had been established for 25 years and they had standards for how long a job should take. If you are anything like us, your installs are completely custom and can take anywhere from 10 minutes to 3 days. We charge our clients per hour and bid how long we think the job should take starting from a 1 hour minimum. Since I'm the newbie in my company (been here 5 years, ha) I'll use me as the example: If I don't finish the job within the time that was bid, we reassess what I'm doing and retrain if I need improvement or reevaluate the amount of time we bid if we were unrealistically low. If I make a mistake - say, I ruin part of a graphic and need to reprint - that does not come out of my check or the client's, but it is a dock when it comes time for a raise (enough mistakes, coachings, retrainings and no improvement = no raise.)
There are days when I seem like the slickest person in the shop because all I do are one banner orders or small weeding jobs, then there are the days when it seems like all I did was one order because it took all day to outfit a fleet of service vehicles. The fairest way you can pay your workers is hourly - just be involved with them: shadow them, train them, set them up with mentors who work faster so they can learn how to be more efficient. A job well done and a steady paycheck is incentive enough as long as the environment is a nurturing one. Throwing them a competitive reward without training them on how to get it will only entice the lazy to be lazier by cutting corners while the hard working and dedicated sit and suffer under your objections to their working speed.
Hope that helps.
