I just used my pass, and they charged the credit card rate with speed pass. So the 10 cents I saved I paid in a higher amount.
If your Speedpass is linked to a rewards or cash back earning card, it's still usually worth it to pay the higher credit price, especially if your credit card allows you to earn 5% on gas purchases. In any case, this offer is still worthwhile at those stations that don't charge more, or that charge less than 10¢ per gallon more, for credit card purchases.
(Some stations that do charge more when you pay by credit card still charge the cash price when you pay with an Exxon Mobil gift card. However, you can't link an Exxon Mobil gift card to your Speedpass.)