I'm not a fan of the plate holder or the stickers, that's just an excuse for a cop to hassle you.
An Asian guy I know used to drive a clapped out 300,000 mile red Acura Integra and got hassled all the time. When he switched to a newer white Mercedes that he bought for $12,000, no more hassles. He drives a lot faster now.
The color and quality of his car was enough to transform him from Asian gangbanger to Asian businessman in the eyes of the cops.
Turbo stickers and Cobb license plate holders make a slightly different looking Fiesta (most cops don't know what a Fiesta ST is) in to some kind of ricer they can impound for modified smog equipment if they can't hang anything else on you.