Roush leaves plenty on the table so dont worry... they are very conservative when it comes to power as they have to warranty that car. No matter what you must have the tune redone if you go changing things... which will void the warranty
you may not need a tune for exhaust for example but you need to have the tune checked after to ensure you are safe, heck I would even stock as you never know how roush tuned it (or what default can tune they stuck on it).
Was yours done via a ford dealership that is allowed to create official roush cars or was yours created directly by roush? figure that out first as well as both are official its just a matter of who did what... they used to do this and I assume they also continue it for roush.
You should be able to push that motor up to around 600-650hp (550rwhp) or so but any more you may wish to consider the GT500 platform.. but the limits of the 5.0 are still being determined.