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You won't get a discount from a GS boutique full stop. Certainly not in the U.S.
ADs in the U.S. have stopped discounting - the ADs worth their salt anyway, such as Topper and Timeless Lux. Other smaller ADs may give a few percent off, but nothing like the 20% they offered me last year when buying a GS. Retail on a SBGN003 is $3200 before tax. The lowest I could find was $3000. Used in almost new condition was trading around $2400 a month or so ago, so it just made more sense to not buy new. Can order them from Japan for around $2600 IIRC.
Edit: I did not buy whatsoever if that wasn't obvious.
I don't think you will get any discount out of the Grand Seiko boutiques without a history of purchases. I think @Soul got discount from a distributor (can't remember which) on his snowflake. Watches of Mayfair seem to offer discount on GS, but don't know if they would source the model.