If you're worried about it, try a panic stop on a slick (wet or dirty or icey) surface in a safe area like a parking lot or a country road and see if you get pedal pulses to confirm that the ABS system is working.
At 80+ mph in a panic stop you may "initially" exceed the capability of the ABS system. Remember, it must sense an impending or actual lockup before it modulates brake pressure. At high speeds this may mean you get some intermittent lock-up. I used to take my corvette up to 100-110mph and STOMP on the brakes for fun; the wheels would intermittently lock-up and release until about 80mph. I can get my 1998 E320 to do what you described at 40mph on dry roads if I stomp down fast enough and hard enough. It's normal; at least my mechanic says so. He also said that below about about 12mph (20kph) that the ABS system may not work well, if at all.
What you have sounds normal. I used to fly combat military jets and their anti-skid systems were very similar, in that the "ABS" sensor at each wheel can only "read" so fast. At higher wheel speeds this means that the tire covers more ground between each "read" cycle and was more likely to lockup. Our cars work similarly.
Hope that helps.