Yes, the Environment Map is the killer. I recall Skuzzy saying that even the most powerful card on the market can run it at max without a drop in the FPS.
As for a 144 Hz monitor, it's simple mathematics: 144/60=2.4. That means your card would have to do 2.4 times the job to keep the frame rate at the top. If you're dropping from 60 to 50, you'd be dropping from 144 to 50 in the same situation, using the same settings.
With a GTX 980 you'd probably be able to keep a three digit frame rate on a 144 Hz monitor at default settings. Those who've been using them can give more exact setting suggestions.