This is correct on paper, but really understates the impact of going from relatively low wattage to higher wattage. It can result in a VERY different tone and feeling in the low end, and can give be the difference between compressed and 'not quite loud enough' and 'open and loud enough', especially when dealing with a relatively low powered head to begin with.
I agree with you, I didn't even notice a difference between 4 and 8ohms with the 1200 watt Walter Woods. However, with these lower powered Micro's, it results in a noticeable volume gain, and a MUCH more pleasant tone when pushed a bit.