Yes, there are penalties for carrying stuff externally, in terms of both drag and stealth qualities. Both of these factors have been studied for F-35 and the impacts are known.
As for the "high-level" ranges, saying that has the same validity as quoting low level ranges for an airliner. The differences between medium and high altitude aren't as big as George would generalise. He's right about the fraction though. But, nobody is concerned about it. A lot of people equate range to capability and it depends on what the context is. If it's attacking something then you can make up for shorter aircraft range with weapon range. If it's an air-to-air mission then quite a lot of the fuel is often burned in the tactics, which might be different for 5th gen, so one shouldn't conflate the two.