I haven't done mine yet, but the one i'm going with is the Harrop. It's pricey but i know a guy with first hand experience who has organised the tuning of quite a few Mustangs and he said the Harrop has produced the highest KW gains. They saw an increase of literally 30kw between it when tuned and the Steeda one the person had on previously because the Steeda has metal and it heat soaks bad (our engines are hot under there for sure).
I think there's a few decent brands out there though. I considered the Roush one but after a bit of research their one doesn't actually seal under the bonnet so it heatsoaks a bit too.
Yeah I heard harrop are good... also herrod is doing a few good packages so I'm trying to decide what option I want to go down. Car is still very low km still as I dont drive it much. Want to put on a few km before I rip it apart...