I know what you mean about increasing fogginess of the intellect over time during dialysis. I was writing a book during dialysis and I found my output in terms of pages per day constantly going down with each day the dialysis continued, until eventually, for the last three months prior to my transplant, I simply could not organize my thoughts well enough to compose any publishable academic text. All that happened with a kt/v of 1.7 to 1.8, which testifies to the inability of even 'good' dialysis to clear the full range of toxins.
Anyone on the verge of switching to higher doses of hemodialysis could preform the simple experiment that I did accidentally on switching over in 2001. Back then I had a serious dependency on Freecell - the solitary card game that comes with the Windows operating system. If you've never tried Freecell it would be best to not start - the point is that each game presents one with a solvable puzzle - the game has tens of thousands starting points and according to Freecell websites all but one game is solvable.
While dialyzing three days a week I could solve typically between three and five games before failing. This is over a period of years - 1997 to 2001. Upon starting daily short duration home hemodialysis on the B Braun I saw my abilities suddenly and unambiguously improve, so that I could regularly do 10 or more games before loosing. In fact my new abilities finally made Freecell boring enough that I lost interest in the game and moved on to other addictive computer challenges.
I put no stock in Kt/V as a marker for optimal dialysis - it is like trying to determine the temperature in a room by measuring the humidity: you can say what the temperature isn't but you can't really say what the temperature is. My belief is that the only way to determine the temperature is to ask a person in the room, just as it is my belief that the only way to decide if the dose of dialysis is enough is to ask the person being dialyzed.
Now obviously, if any one of us entered a room and then were asked what we thought the temperature was we'd all have a different answer but over time we could learn to distinguish the temperature more and more finely if we were allowed to experience a range of rooms with different temperatures. So it is with dialysis. After the frigid experience of predialysis any room will feel warmer but if we are allowed to feel what a properly heated room is like we'll quickly realize how cold it was in the three day a week room.
On exercise I can relate my experience in 1995 when I joined the carpenter's union after 3 dialysis years unemployed. I worked setting up trade shows and as low man on the totem pole I was given many of the unpleasant, physically demanding jobs. No one goes to the gym for eight hour workouts but that is what a shift was like in 1995 to 1998 when I moved into making graphics most of the time. The more I worked, the better my appetite was, the better I would eat, the better I would sleep which then meant the more I could work. I turned the downward spiral I was experiencing in 1994 into an upward spiral. Vigorous exercise made a huge difference in my life and how I perceived dialysis.