My argument is that a given airfoil makes a certain amount of lift at a given speed. So long as the foil is making enough lift to counteract the weight of the plane then no addition angle of attack is required to keep it in level flight and therefore no extra drag is created. If the foil doesn't make enough lift to counteract weight then some angle of attack is required to keep the aircraft level. In this case the more the aircraft weighs the higher the angle of attack required to keep it level and hence more drag will be experienced. So in this case you would be correct that a heavier plane will be slower. However conversely if an aircraft were to create more lift than is required to stay in level flight then a nose down condition will have to be used to keep the aircraft from climbing. This negative angle of attack would also induce drag. I'm not an expert in the field so this is all coming from my head and I could be very wrong. I'm not sure how much the excess lift in a nose down condition would attribute to forward motion though I doubt it would cancel the drag created. I will do some reading an inform myself
. Interesting subject so I'm sure it's been discussed elsewhere.