If the mapper is saying you MUST have a speed sensor then they don't sound like a very confident mapper. My mapper told me the boost and turbo speed on these new G-series turbos is so predictable that I wouldn't need one. He said you can use a speed sensor to help getting the boost control perfect at the right rpm but because the turbo is so predictable you can do it without.

im having one on mine as a safety feature as my aftermarket Ecu can control it and can cut boost pressure if it sees a boost leak so no issues of over spinning the turbo but I don't need it for the purpose of mapping.