Hey Steve,
Waterbase or discharge does work well on pique... One issue you may find is that you will not get a full reaction from the discharge on those sport grey colored shirts due to the poly content in them, therefore, your colors would not be completely opaque and color matching would be very hard to hit. A black water base print on them should be fine, most colors of ink darker than the shirt color should be fine, but if you're trying to color match lighter colors on that garment, you'll likely have issues. You should explain the process clearly to the customer, let them know you can do it with water base, but let them know the limitations... Another quick fix would be to get them off those athletic colors, and get them to go with a silver or a charcoal color or something...
