First of all, these cars are going to be reliable. Trips to the dealer for service should be pretty rare.
Regarding the warranty, most of the mods currently offered will not void the warranty. However it's a little more complex than that. For details look up the Magnuson-Moss Warranty Act. The short version is that the dealer has to show that the modification caused the failure before they can deny a claim. So as an extreme example, they can't refuse to fix a wheel bearing because you have an upgraded Diverter valve. However if you have a bigger turbo and blow a hole in a piston, it would be reasonable for the dealer to blame that on the mod so they could deny that single claim, although they still couldn't "void" the entire warranty.
Now the tricky parts comes in when you have a dishonest dealer. They can try and bully you and say that a mod caused the problem when they know darn well it didn't. To avoid that whole debate with them, in the event of a bigger claim you can return the car to stock.
Here are specifics for Eurocompulsion products.
1. Intakes. No claim has ever been denied due to one of our intakes. In fact at least two Fiat dealers install our intakes and carry them in their parts department. There isn't anything to worry about here, but if you are worried, the stock intake can be reinstalled easily and quickly.
2. Euro+Drive Lite. It's not going to cause any engine problems,and can be removed. Once removed it's IMPOSSIBLE for anyone to tell it was ever installed on the car. Even when installed it can be very discrete and difficult to detect, especially if you use the bypass plug because it's very small.
3. GFB valve. I have heard of a couple dealers making an issue, but in both cases they changed their minds. That valve really doesn't change anything, it operates exactly as the stock valve does, it's just a higher quality longer lasting part. So there is no case where it should cause a warranty issue provided the dealer understands what it does.
4. Big turbo. They are going to notice this and deny any claims for a damaged engine.
5. Performance Camshaft. They won't be able to detect it without a great deal of effort. If they do detect it I suspect they would deny a claim for any valve related issues.