I love the look of the knife and profile of the blade. The taper "issue" is one that I feel you'd have to experience firsthand, so I can't comment on it.
Now, would I buy a knife that is hardened to 64.5? Yes, I would.
Would I notice an advantage either at home or the hall, where I put my knives through the biggest pounding? Maybe.
Would I request it? No, not in my situation. However, if I were a line cook/chef at a restaurant, I'd definitely consider it.
With that being said, I'm more interested in performance and ease of use. It seems to me that Salty was impressed with your grinding abilities and skills as a maker. To me, that's huge.
The steel both has me intrigued and makes me yearn for classic, old-school steel that gets wicked sharp, patinates like an SOB and just has that charm about it. I might be one of the most confusing guys out there in regards to steel preference. I love learning about new super steels, and I love my S35VN that you made for me in the same way I love my 1084 Mike Davis made, and my white 1 by Murray.
Chasing the newest, greatest steel can be quite a ride, and often it pays off. However, there's something to be said for a proven winner. Since we allhave different tastes, and it seems as though you did a great job with the steel, I'd say keep it as an option.
Personally, I'd likely choose O1 or W2 at 60 and love the hell out of it, while a guy like OD would likely go with "SuperBetaW5", in hopes of having the "perfect" steel.
Regardless, I appreciate your being so candid and letting us in on a knife we were (and still are) all captivated by.
I have used a stainless bladek hardened to HRC 64.5 years ago - an Akifusa Gyuto - and even being a lesser sharpener back then, and stainless, it wasn't too tedious on the stones. It didn't feel great on the stones, like i presume your carbon would, but it was ok. And I agree with a lot of others, the comfort allure and performance of most knives should be in their blade profile, geometry and grind. With that said, extreme HRC would be welcomed by people like me, working the line every day.