The non-factory belt issue is an often overlooked and fortunately easy to solve problem. Apparently there is slight slippage which causes a problem with the crank position sensor reading accurately, which throws the misfire MIL.
To check you belt tension, Napa auto parts sells an $11 Kriket belt tension checker which is identical to the Porsche tool.
Item#: NBHKR1
Price: $10.08