Bottling from Keg. Long term aging question.

I've got some Imperial Porter that I'd like to last longer than it'll last in my kegerator. Should I prime the part that I'm bottling with priming sugar or is there a good long term storage option for bottling from the keg? I'm familiar with counter pressure fillers and the beer gun, but was wondering how the beers hold up over a period of maybe 6 to 9 months.

A properly carbed and properly capped, bottled beer should stay properly carbed for...a very long time. Doesn't matter whether it was bottle-conditioned or BMBFd or Beer Gunned. On the order of years at least. 6-9 months is nothing imo.

I think the perception that BMBFd or Beer Gunned bottles lose carb over time is because when you cap that bottle there is no pressure in the headspace of the bottle. CO2 offgases from the beer to pressurize the headspace, so that little bit of CO2 is no longer in solution. The bigger the headspace the more CO2 is lost to pressurize that larger volume. That's why people who bottle from kegs often leave very little headspace in the bottle. But this lost CO2 happens fairly quickly, on the order of days or less. After that it should stay the same.

FWIW, when I bottle from a keg I chill the keg cooler than I would serve it (usually down to 34-35 F) and let it get a tiny bit over-carbed, chill the bottles as cold as possible prior to filling (no frost though), and leave very little headspace. By the time I lose a little carb to the bottling process and a little more to the headspace it's about just right.

EDIT: but make sure you leave some headspace. If not then just the expansion of the (non-compressible) liquid as it warms could cause a bottle-bomb or at least a leaky cap.