In 10-15 years a prospective buyer for these cars are going to want a Fiesta ST to hoon and modify. They aren't going to give a hot shite about an old expensive factory option that gave a marginal boost in power,legal or otherwise. I can say if I was buying my fiesta again I'd take a Recaro Green Envy over a non Recaro white/magnetic/black one with a mountune package. No disrespect intended to those good colors of course .
In regards to the cars future desirability I see it becoming a future Suzuki Swift GTi. The Fiesta ST is very similar to the GTi in terms of its underdog quality and its unapologetic need to have fun at all times. I own a modified restored example that has a small community of devoted members. When GTi's come up for sale they are quickly snatched up and are worth 4 times what a regular Metro goes for. That being said they aren't ultra valuable and even my nicely restored one would probably sell for less than $5000.
Buy a car for its current value and character, not for what it might sell for decades in the future.
View attachment 23957