My Porsche Turbo 996 with X50 and manual 6-speed box and aerokit front spoiler has just been revalued for insurance. The price is holding very very well where people talk of a softening in prices. As for the GranSport, I think their value is going to be in the rarity and scarcity of numbers. They may have taken a recent slump in price, but I think that will pick up due to the low numbers. The reason why GT RS and GT3's are still climbing the value ladder is down to scarcity in numbers. Porsche 911 and water-cooled front-engined cars are still climbing in prices -maybe they are just better marketed as a brand?
Mine is a last year (07) LE car with low miles and brilliant service history - I have owned it for 5 years and it goes to Marios at Autoshield every year for service and till now, not a single MOT advisory. At the silly prices being banded around, I'll just enjoy it and run it. I see myself at some going into a facelift GranTurismo, but the cost to change is ridiculously expensive for a car in high numbers that will depreciate over time. Predicting the future is fraught with difficulty, so I just focus on enjoying my two cars - both impeccable low mileage highly desirable models and good clean cars with good service history and maintenance records.