Most of the time, the vouchers do save you a little bit of $$, but with GK breakfast, you will always lose $$ with the voucher. You are better off using the AP discount.
GK breakfast is $30 for adults...with tax and tip the total is $36.90. The premium character meal voucher is $44 (even with the LMT discount...maybe they charge $42...you are still losing $).
Remember that in order to use the discount, the AP holder must make the purchase. You may want to make sure you have cash or Disney Gift Cards your family members can use to pay for your meals if you want to use their discount.