Just got back from abenteuer allrad in Germany and saw the KCT and Outbound kit first hand. The KCT stuff is luxury yacht standard, but to be honest I actually preferred the construction and look of the Outbound kit. The frameless double glazed window and slimmer profile frame looked so much tidier than the bulky KCT stuff. The seitz s4 windows are so cheap and plasticcy in comparison, but I can't fault anyone choosing them (as I likely will!) given the cost of the alternatives.
What is the price difference between the Outbound and Seitz (sorry, havent looked at seitz pricing)? I've not installed the Seitz but have seen them in a few campers. The Outbound feels like a pretty big step up in quality and durability is my laymans impression, might justify the price difference. I've handled a set of KCTs before, and they are as you say, luxury yacht level. And luxury yacht weight. I think unless I was building a Unicat level camper, the KCT would be overkill.