You car looks awesome man, sleeper hmmm ?? not sure.
As to the sleeper thing...depends on who you are trying to fool.
To get over on another mustang fan the car would have to be bone stock looking from the outside, no guages showing on the A piller or dash, no loud exhuast, nothing more than maybe lowering it...but even that is going to make most mustang people double check ya out...before they jump on you. The car would almost have to tricked out as a V6 to really fool someone(a mustang person).
Now for everyone else other that other mustangs, no badges, no big hood, no loud exhaust, same recommed trim it out as a V6... remember camaro and firebird people are car people too...so if you are in an obiviously GT/5.0 they are going to check you out too, if you look too mean they might just let you go.