This is a little focusing nozzle I made for my M18 Fuel Leaf Blower.
I can't say it dramatically increases the air speed, but I figured I'd post my attempt anyway. I think it probably doesn't work the way I thought because the M18 Fuel Blower uses a standard fan to force air. Where some other blowers use a centrifugal impeller.
The fan produces a high velocity, low pressure stream (fine for this application). Where a centrifugal impeller would produce a higher pressure stream, allowing for a nozzle to accelerate the air.