It does not make it more efficient, that is a bald faced lie. All the goodman units I have ever seen (And as a common brand, I see a lot of them), have a pston type metering device. These have the Freon pressures equalze in the off cycle. The only reasons you would need a hard start kit is with a low voltage situation or if the compressor bearings are getting tight. If he is installing it that easily he is using a Superboost, which is not a true hard start kit, but is a start assist. A true hard start kit has a start capacitor and a start relay wired to the compressor. Yeah, the selling price for start assists around here is $25.00 to $75.00 plus labor, depending on the exact model.
Geta new contractor. He will keep feeding you lines till he gets your money. You can bet the tech is on commission.
Stretch