When the pressure plate is tightened down on the mirror, the mirror does not move relative to the central baffle from that point on.
The tape would aid in proper centering of the mirror on the baffle, but it would have absolutely no effect on slop in the focuser.
The slop in the focuser is almost entirely due to the clearance between the outer baffle, to which the mirror is attached, and the inner baffle attached to the back cell of the scope.
There is thick grease in between these two baffles, and the outer baffle slides up and down the inner baffle as you focus.
So, what can be done to lessen the slop that causes image shift?
1) Thicker grease. if the grease does not flow away from the pressure points as one baffle is tipped relative to the other, this will reduce image shift. Such a super thick grease might not work well at sub-zero temperatures, though.
2) tighter tolerances between the inner baffle O.D. and the outer baffle I.D. Ball bearings running in a track would have done it, but would have increased the production costs.
a smaller gap would have been hard to grease and could result in metal-on-metal scraping.
3) a strong spring behind the primary mirror around the baffle that presses the mirror forward. This might lessen image shift, but it would also mean the focuser would be harder to move in one direction than the other.
4) Having the pressure to change focus come from the center instead of the side (e.g. helical focuser, center-mounted).. Some SCTs did have this.
5) A fixed mirror with a rear-mounted focuser. This was deemed unacceptable because it would significantly restrict focusability. Current SCTs focus over a very wide range from near to infinity with eyepieces, cameras, reducers, different sizes of star diagonals inserted or removed, off-axis guiders, etc. A rear-mount focuser only would limit the usage of the scopes.
There is one simple thing every user could do that would help. Every time the scope is used, run the focuser from one end to the other about a half dozen times to get the grease well-distributed. It will reduce image shift. Then, once a year, remove the focuser and use a pencil eraser through the hole in the back to turn the primary mirror around in a circle several times. This will distribute the grease uniformly over the interface between the two baffles. Then, grease the focuser threads and reinstall the focuser.
In the C90, heavy grease of suitability over the -50 to +150 temperature range goes a long way toward making focusing smooth. If the focus is central (i.e. helical), that's all you need. There's no harm in centering the mirror, however.