I doubt that a claim that federal law trumps state law is going to hold any water. Federal law works fine if the feds are after you (think marijuana laws), but the state and local laws are going to be what local police are going to enforce. Just look at what is happening with ebikes in NYC.
To the original question, no one is going to hassle you if you are not riding above 20mph, and are not pissing people off. A small 250-500watt hub motor below 20mph is fairly inconspicouls and isn't going to bother anyone under normal circumstances (assuming you are not intoxicated - then all bets are off).
Technically for federal law, the bike should not go more than 20mph on its own, but can go faster with pedal assist. That way the "specialized turbo" (or my bike which has similar specs) can be legal even though a good rider can do mid to upper 20's on it.