Here is my two cents on it. Main point: more exciting drive, very expensive upgrades/carbon, and a rarity premium all make the trade interesting if you find one you really like.
1. Ask most who have driven/owned both and you will find the driving experience is very different between the 16m and regular f430 spider. Not only more focused, but the SuperFast 2 transmission makes a huge difference and although the headline power increase does not seem huge--it comes much lower and is very usable. I have driven the scud and 458 on the track and even prefer the scud over the 458 as it feels quite special and is more fun in my view. I think the lap times would be close and for a non-expert driver they would most likely be faster in a 458. The scud/16m, though, feel super special and are a real "event" to drive.
2. The cost of the upgrades vs the f430 is a really big number. You can do the math yourself if you consider all the carbon fiber pieces, engine upgrades, transmission, racing seats, titanium bolts, wheels, etc etc you will get to a very large number.
3. The 16m is a rare car. There will be way fewer for sale at any given time than the f430 spider and for "special v8" collectors there will always be some demand and additional premium compared to the normal model. So--part of the price difference will not be lost. I'll leave it up to you to calculate how much.