Unfortunately, even if a lab goes off the GS scale, the top pay rate allowed by law is still the same as a GS-15 step 10. In the DC area in 2017 that was $166k and change, including locality pay. The only thing using a demo pay scale helps with is easier promotions, which is great until you get salary capped.

$166k/year isn't bad, by any means. But it isn't competitive for a PhD scientist or engineer with equivalent experience.