Well it wont be much of a saving when fuel is $8 per litre will it.
One half smart would give a lt per hr figure.
8 x 500 hly mag inspections would be required on a 2,000 TBO (TBO is the norm for working out costs).
More like $40 per aviation massive electrode spark plug.
More like $20 for good quality automotive spark plug.
Hidden costs for the Pmag are a big problem (accom and freight such as TNT).
O/H exchange slicks are $900 AUD and weigh 3 3/4 pounds (about half the old slicks).
15 mins Pmag change on a new Cessna 182 (as per the OP aircraft) take more than that to obtain the EO/STC paperwork. The new Cessna's are very tight in the engine bay, you need calibrated tooling and possibly special tooling for the Pmag ( I don't know, might take me an hour to research it depending on the net!). take of cowls and do required paperwork! If you meant an extra 15 mins - only if the required documentation such as the EO or STC and any required test equipment is supplied and confirmed can be used. Some maintenance places require these to be incorporated into systems and registers before use. Most take more than 5 mins to read and understand.
Fuel burns at a constant speed, while variable timing can help to burn more fuel at a given RPM to optimise that - I doubt that the difference at cruise is much like $8 per hour (working on $2 per ltr).