Here is some real world experience between cordless and pneumatic impacts. The pneumatic impact will have more grunt then a cordless with the same specs.
My heavy duty snap on impact is rated for 800 tightening torque, 1100 ft/lbs breaking force.
My Mac cordless was advertised as having 1400 ft/lbs breaking force, 1100 ft/lbs tightening.
I called BS as soon as I saw the specs on the Mac truck. And pointed out there specs were way off. I then talked the tool man on the truck, as well as his boss who was riding with him into a quick demo in the shop.
It was the tool man's first week on his route, second day on the job
I took a loose 3/4" bolt on a flange and tightened the nut with my Snap on impact. Then loosened it with the cordless impact. The cordless struggled for a couple seconds before the nut finally broke loose.
The tightened the nut with the cordless hammering it as tight as it would go, and broke it loose with the pneumatic impact. The pneumatic impact never even sputtered before backing the nut off
I still bought the cordless impact though. I was impressed with it
Just pointed out to the tool man that the battery operated ones won't out do a good quality pneumatic impact