I liked the idea of them, but the fibres shrink after a couple of days (the pull due to the curve of the mirror cap), it became a PITA. The way to get around this is to wrap the mirror in two parts of vinyl. But it can look a little messy.
I gave up in the end and turned to gloss black instead which I think looks better (on red at least and may work on white):
I've had my 8p a3 car wrapped in the past and I wouldn't recommend it. The wing mirrors look really messy once they're wrapped, they can't hide where it overlaps. And that's the bit you see when you look at them from inside the car!
An expensive alternative is to get the replacement caps off eBay, over £300 but will look factory good.
Otherwise, leave them alone - they do look fine as they are!