Okay, I might be showing my noobness here, but seriously how much more can a 15k pin appreciate. You already have a very, very small segment that can and would pay that much for a pin. It is obviously great looking and must be a pretty good player considering the rankings. But IMO when LCDs become the norm on pinball in the next year, 2, 5 or whatever I just don't see the price going considerably higher.
Now if the investment is not the real issue and you love the pin and love the rarity factor then I would say keep it.