This is my take on aftermarket MAFs. I was friends with an engineer on Ford's MAF production line, mostly a go-between for the plant and the designers, so I know a little about how they work.
- Recalibrated units with their own electronics are fine (Pro-M included), as long as they are properly calibrated for your engine configuration. I suspect Keith's is not.
- Units that use your stock electronics with a "flow tube" are less desirable. The electronics are hot calibrated on a flow bench in process (at Ford) after the electronics and the sensing elements are fully installed. I'm a little wary of anything that requires you to move the electronics. Same logic goes for removing the electronics to clean them (or any other reason). There is a reason Ford sells them as a unit. The electronics don't even have their own seperate Ford Part Number. Well, they do, an assembly number but not any end-item part number. Nothing that is meaningful outside the production line or engineering.
- Don't go hacking your stock MAF to make it flow better. I'll leave it at that.
EDIT - OMG, that was my 3000th post. I need something else to do in my free time.