Spiffs are a short term solution for a long term problem and I use them very sparingly. I don't know which is worse, the occasional super salesman or the expectation of always being paid a spiff.
I believe that the pay plans should be designed to produce the results you are looking for as Rykid alluded to. Tiered pay scales for production technicians and different rates for maintenance work. If they work in a team environment all members must be close in hours turned to get bonuses.
Advisers would have tiers for hours per R.O., parts to labor ratio, and percentage of additional lines. I always pay for each tire sold with a higher value if it is sold during the walk-around.
I give high pay rates but I put bonuses in the pay plans as if they were an NFL player: CSI, training, most tires sold, highest hours per R.O., no complaints, no bad comments on CSI reports, no bad Dealer Rater posts, clean bays/work areas, highest parts/labor ratio, no vehicle damage, highest profit per R.O., and many more.
It's been my experience that spiffs accomplish two things; higher payrolls and customers complaining about being oversold. Just my 2¢ worth.