That's running, I don't think a low impact elliptical will get yo those numbers. 7.5 min mile is fast!
I've rotated through using the elliptical, treadmill and stairmaster at the gym. I change it up based on boredom.
When I train cardio, I do interval training of 60 seconds as hard as I can go, 90-120 seconds 'jogging' pace depending on how I'm feeling and how much recovery I think I need to hit it hard again for 60 seconds. Training sessions are always 30 minutes.
From a typical 30 minute session I get calories burned readouts of approximately:
550 elliptical
475 stairmaster
450 treadmill
in terms of which machine I feel kicks my ass the most
Stairmaster - and by a large margin
Treadmill
Elliptical - definitely the easiest
That's the reason I place little stock in how many calories a machine tells me I've burned.