Ok to be perfectly honest I have a 2003 Ranger with the same 4.0L V6 as the 2005 mustang. I am wondering are there any aftermarket parts for the 2005 mustang V6 yet ?
Check out the Sport Trac website for aftermarket parts. I think the Sport Trac has the same 4.0 L engine. This website has some of the best articles, people, and discussions. I traded an 03 Adrenalin edition Sport Trac for my 05 GT. It had an Apten performance chip and a Mac Intake. It ran great for what it had. One note; Stangnet seems to be a great website, but the function and format on the ST website seems to work better. Just my two cents...
I doubt it will be a huge thing, most companies don't make an aftermarket for the secondary engine, they build for the most powerful one. I guess the best aftermarket would be a new engine