As speed rises, the smoke wants to adhere to and follow the contours of the boiler on steamers with low back pressure, which the best modern ones had. The smoke lifters did not lift the smoke, but they increased the laminar flow ALONG the boiler at speed. This denser laminar layer prevented so much penetration by the smoke. It was meant to keep the smoke from licking around all the appliances, domes, and piping to the point where it effectively obscured the vision through the forward glass by both the engineer and the fireman.
If you watch youtube videos showing UP 844 at track speed, there is lots of smoke drifting back, laid flat along the top of the boiler at times, depending on conditions. However, you virtually never see the smoke drifting down the sides of the boiler and going past or through the cab windows.