Don't worry, the problem only gets worse as the car gets older. Mine has had this issue for over a year and has been into the OPC approx 25 times (no joke). It is not a stumble, it is an actual misfire that is not logged as a fault. This was found by local OPC using a test program from factory. It also starts to affect the lower rev range performance wise before coming onto cam, they may also have found that one bank was running rich, and the other lean. Oh, the stutter where the car all but stalls is really good fun, especially when you pull out of a junction quickly.
Mine has covered 46k now and is a 2012 Carrera S, here is a short list of the stuff I can remember being done.
Every ecu module erased back to factory and upgraded, multiple times.
Crank sensor spacer fitted, then later removed.
Another software update
Another software reset
PDK reset (gear change was clumsy due to poor engine running)
PAG Software re-write
VVT got stuck in high lift on one side, giving the most awesome exhaust drone, and high pressure coming out of 1 of the four exhausts)
VVT and VVL replaced on 1 bank of engine
Software reset
Car then droned when moving on and off cam, so loud you could hear it at 70mph
VVT and VVL replaced on other bank (they had the car for 3 weeks that time)
Has it been fixed? No, The problem goes, and then gets worse. Sit in your car at idle, does it feel like it misses a beat every now and again, the car physically rocks.
New features like windows unresponsive, pressing sport and the light comes on but it stays in normal, auto stop start deactivating on its own (shown as an error), parking sensors coming up completely red and then showing "park assist error deactivated", clutch being banged in and out multiple times when pulling away makes me think this is a can bus related error, but the good news is that critical systems such as engine management, air bags, brake lights, all share the same bus, so when the brake lights don't come on and I get crippled by the lorry of death that hits me because the air bag didn't inflate, then at least I know that it all started with a niggle between 2-3k revs...
Luckily mine is leased and I only have another 8 months left to go.
I am presently writing an article on my "Porsche Experience" which I am going to send to every Car journalist I can find. If anyone would like to contribute to what I am writing, I will gladly add it in.
ps, glad you are enjoying your 991's, great cars aren't they....
