Air Flow Meter Mod - Questions

I have just bought an Air Flow Meter Mod from Ebay. It's a simple resistor that fits to a break in one of the AFM split wires.
Does anyone know of any adverse effects this can have on the car?
Does it effect the MPG?
Does it invalidate insurance?
They say it is easy to remove for servicing/warranty jobs but......
Does it need to be removed for standard Ford servicing? Is the AFM ever opened?
Is it not obvious when removed as surely the wire is just twist connected and covered in insulating tape?
Thanks in advance for any help
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