I just purchased the "Active Rider" winter boot. Though I have not yet had the chance to ride in them yet, from trying them on, they feel very, very similar to my Sportive High Rider II's (the non-winter dress boot). I am really impressed with how relatively streamlined they are.
As for the calf width, I do find that Mountain Horse boots tend to be significantly wider than most. I wear a wide in most tall boots, even an XW in a few, but I can easily get away with the standard calf in Mountain Horse boots. Like you, though, I was concerned about the added thickness of winter fleece breeches and whatnot, so I went with the wides for my winter boots.