That's very odd behavior. Wheels that are a few pounds heavier should not effect a car with this much torque, or clamping force. Do you happen to know the exact weight difference including tires? You've moved more tire mass towards the outside, which at high speed will certainly be harder to bring to a stop, but if little econobox Hondas can run around with 20s all day, I doubt that is your problem.
Did you ensure the tire pressures are not jacked up to 40 lbs like they do at the mounting place? They will also need a few hundred miles to break in. If they are a hard compound, for longevity, it could be just a lack of grip compared to old tires. It's amazing to me when I upgrade tires on cheap non-performance cars how much more effortless the brakes get even at highway speeds. There'a a whole lot of friction going on at any speed.
