Steeda's axle back sounds great, but isn't too annoyingly loud. Up to 10 RWHP increase. Up to 20% weight reduction
Sound clips
https://www.steeda.com/steeda-555-3612-s550-axle-back
Looks good on a Bullitt as well
I have never heard of Winjets. Form is a sister company of Diode Dynamics, which makes good stuff if you ask me. I have DD turn signals and side quarter lights and Form headlights and smoked tail lights. They all tie in and work well together. Have not had any issues with fitment, fogging, etc.
If it's not worth the work and effort, just buy a new one, they're not that bad.
https://www.steeda.com/10-1015-101-mustang-smooth-rear-decklid-panel-gloss-black