Hi Mats--Welcome to EpicSki!
Stiffer or not, you will most certainly benefit from a high-performance boot and a custom fit and setup. The boot is the most critical component of your equipment, as it's the piece that attaches directly to you.
In general, for serious skiers (that is, for skiers who are, or are willing to work toward becoming, advanced or expert skiers), I recommend at least moderately stiff, high-performance boots. It is possible to go too stiff, and you may not enjoy those stiffer boots right away. They will communicate your movements more directly to your skis--both good movements and bad. They reward good technique, but are less forgiving of technical errors. They'll "force" you to become a better skier!
And stiffer boots absolutely must be set up correctly. They must fit well, of course, and being typicaly snug, that fit can take some work. But even more importantly, they must be optimized for you--including both lateral canting and (especially) fore-aft setup. Nothing skis better than a well-set-up high performance boot. And nothing skis worse than a poorly set up high performance boot. Without expert setup, you'd be better off with a softer, lower-performance boot.
Don't hesitate to post your question in our Boot Gurus forum, for some real expert advice on both the type of boot that might work best for you, and the importance of proper setup.
Edit--I see now that you have posted your question as well in the "Ask the Boot Guys" forum. Good work. You'll get good advice there!