So, I’ll give you my opinions, but take them with a grain of salt, and also maybe consider demoing something similar first.
So I wear sz 9 boots. I ride all my decks at 26.0 in the waist. They don’t turn slow, not at all. For one I get the Carbon construction which makes the decks nice and snappy, then in addition to that i ride stiff responsive bindings along with stiff boots.
So why are you getting a Khyber? Maybe that is a question you need to ask yourself. They make pow decks wider and then with taper for a reason. Wider with more surface area makes the deck float better, narrowing the waist will counter that. The taper does two things, it helps with float by allowing the tail to sink, and then it helps make the deck turn easy, a tapered tail will really release easy into turns.
So if you mess with less taper and less waist width the board you get will not ride to it’s full potential as it was intended, that doesn’t mean it won’t ride well, per say, but it won’t ride quite like it would have.
I made changes to my deck, yes, but I was on the R&D program of a major MFG for quite a lot of years and was in the industry for a long time and have literally ridden hundreds and hundreds of decks, and with being on the R&D program ridden a lot of prototype decks and many times would get 3 or 4 versions of the same deck at a time to try out and to notice the subtle differences with and to be able to feel exactly what those subtle changes in specs would equate to on snow.
That explains why my custom build experience with Prior went differently than the other poster on this thread.
When I called I knew exaxctly what I wanted, I laid out all of the specs and told them to build it and put a Khyber nose/tail on it.
I can tell you from experience, and I’m willing to bet you a swift kick to the balls, that the deck you seek, will turn much better with a pair of stiff boots and bindings then it would by narrowing the waist 7mm. You would also then be retaining the float afforded by “extra” width.
Obviously, it’s different strokes for different folks, but, as I stated in the begening of this reply, maybe you should get out on a tapered pow deck to see how they ride before you go droppping $$$ on custom specs. Because if you have no frame of reference to how it rides to begin with, how do you know what you are doing to it is gonna make it ride any better???
YMMV
FYI, you spoke of how you like how your 148 board turned better than your 152....
I can tell you, this could be due to waist width, OR it could not have anything to do with it.
I could literally give you two 152cm decks with the same waist width and could give you one that turns way quicker than the other.
Waist width is such a small, small, small, part of the equation. The type of core, type of glass, the bias of the glass, torsional rigidity, sidecut, effective edge, running length, stiffness, type of and/or amount of camber.... All of those things will equal a much greater sum when it comes to how a deck turns, then the single sum of waist width... Then factor in a pair actually stiff binding/boot combo and waist width doesnt really mean hardly anything with how either on of those identical waist width 152cm decks will turn...
While making snowboards isn’t exactly rocket science, there’s quite a bit more to it than, “I want something that’ll float, but that isn’t too wide (chattery and less maneuverable).”