It's relative, so your characterizations for low speed are just fine, i.e., 3 - 8mph is low speed. High speed is dependent on what you're doing. If your diving through right-hand or left hand turns, even 15mph could be "high-speed" in terms of how well a tandem handles when heeled-over at a steep lean angle.
Bear in mind, these characterizations are meant to explain initial impressions, all of which are highly subjective for any one of a variety of reasons (To make an analogy - Do you like your steak medium rare, medium or well done?). Morover, most tandem captains / teams who take up tandeming on regular basis quickly adapt to how most tandems handle by the second or third ride and their inherent handling characteristics become "normal". You don't often appreciate (or come to lust-after something new and different) until you take a test ride on a different tandem after becoming accustomed to the one you regularly ride.
Again, case in point, anyone taking either of our tandems out for a test ride (which is a small population given how small they are) is usually put-off by how "twitchy" they are at low speeds, e.g, starting out and climbing steep hills. They also quickly discover how much their stoker moves around and re-institute the "tell me when you're going reach down and grab for a waterbottle" rule. However, after the first 40+ mph descent or diving through a corner heeled over ala personal race bike an evil grin appears on the captain's face...
Not everyone will appreciate the trade-offs between high vs low-speed handling characteristics, particularly teams with larger stokers. The higher the stoker's CG, the more pronounced "stoker-induced steering" becomes on long-trail tandems (remembering, this is why steering trail is reduced on personal bikes as the frames get larger). Also, teams where the stoker isn't predisposed to enjoy steep lean angles or blistering fast descents may or may not reap the benefits of the longer trail.
Again, the take-away from these postings is, initial impressions are not always the most reliable ones nor can you assume that what one team or dealer thinks is "great" will be great for all teams. Therefore, go into your tandem test rides with an open mind and remember that only you can decide what brand or model of tandem is the "best for you". Knowing which tandems have long trail vs. short can help explain some of the handling differences, but not all. It's just one factor. Moreover, if you take up tandeming you may find that your performance expectations of your tandem change and something different might be called for: that's why they're called your "first" tandems.