I know a little bit about 5.4 building, not near as much as I should but a little bit.
It's definitely true that the long stroke of the 5.4 modular is not ideal for high rpm, and even complete race block builds by al papito don't like the high side of 9k rpms so yes that distinction is definitely a valid one against the 8k rpm BMW v8.
That said though, a 5.8l at a healthy and feasible 7000-7150rpms (7300+ is scary on relatively production class 5.4's) could easily make 500hp considering a that the BMW motor is making 414-450hp with only 4.0-4.4L's.
If ford shot for only 86.5hp/L, it could make 500hp safely all day long with a meaty torque curve. If they shot for the same efficiency as the boss 5.0(444hp/88.8hp/L which is unrealistic as that spins 7500rpm) it could make 515hp.
Considering ford has the motor spinning to 7k already in gt500 supercharged specification, it possible they could get an n/a version to spin to 7250-7500 and make 500hp.
Making 500hp n/a in this theoretical 7k+ revving engine is no problem, but designing the right intake and so it doesn't have compromised torque in the useable mid-upper range would be tricky, IMO, without something variable like IMRCs. :shrug: Bench racing is fun, but I doubt Ford will go this way.