That is COMPLETELY false.

You can tune fixed blade heads to shoot accurately at virtually ANY speed......it all comes down to bow and arrow tuning.
If you take the time to do it right they'll fly just fine.
Now some heads seem to fly better than others, and getting them to fly properly gets a little touchier at high speeds,(I'd hardly call 250fps too fast!)
But to say that you CAN'T do it?......sounds to me like you need a new pro-shop! What mechanical head were they pushing?