I definitely think the Bullitt would benefit from the Roush suspension/power add-ons, But I'm not sure if throwing a big ole body kit (even as nice as the Roush kits are) would be the way to go. The Bullitt was all about being understated; a sleeper. The stock look of them certainly pulls that off well. Even among the Bullitt owners I know and have spoken to, the 08's are love it or hate it. Myself, I like it, but agree it could use visual aids... If it were ME, I like your idea of the GT500 front bumper. and alternative to that, in order to keep the stock headlights, would be to throw a Saleen Heritage bumper cover on the front. I know Saleen is probably a bad word around here, lol, but that bumper is essentially the Shelby GT500 bumper, minus the slanted uprights, so you can keep the stock headlights. I could see doing the window louvers too, but I would shy away from a spoiler or hood scoop. MAYBE a small ducktail spoiler would work, but I don't see a spoiler fitting the true Bullitt theme. As for the hood, one of the most common/practical things to do is to put a twin vented heat extractor style hood on the Bullitts, since that is what the GT390 from the movie had on it.
fwiw, my 428R Bullitt tribute I was going to work on would only have received the seats and steering wheel out of the '08 Bullitt...
And DHG paint, if I couldn't get the back one i wanted. (I have a thing for black Ford vehicles, lol)