In a full-rudder sideslip in a Schweizer 2-22 or 2-33 glider, is the descent rate higher at 50 mph airspeed than at 60 mph airspeed? Is it really true that in a full-rudder sideslip in a Schweizer 2-22 or 2-33 glider, the descent rate is higher at 50 mph airspeed than at 60 mph airspeed?
Likewise, is it really true that in a full-rudder sideslip in a Schweizer 2-22 or 2-33 glider, the no-wind glide path is steeper at 50 mph airspeed than at 60 mph airspeed?
I've heard several instructors in my glider club assert these things.  Some allege that at higher airspeeds the fin overpowers the rudder and reduces the obtainable slip angle (as measured by the yaw string).
I've logged plenty of time in these types of gliders but never have attempted to explore this issue experimentally.  But my intuition is that the slip angle (as measured at the yaw string) is not significantly larger at a lower airspeed than a higher airspeed, and that a given slip angle (as measured at the yaw string) produces much more drag (and also much more sideforce, requiring a larger bank angle to compensate) at a higher airspeed than at a lower airspeed.  Therefore I would expect the descent rate to be higher at a higher airspeed, and I would also expect the glide path to be steeper at a higher airspeed.
The minimum sink rate airspeed and the best L/D airspeed in these gliders are both in the neighborhood of 40 mph.
Bear in mind that the no-wind glide path is determined by the ratio of Lift to Drag, which is very nearly equal to the ratio of Weight to Drag.  So the question about the glide path is simply a question about which configuration creates more Drag.  Also, it is clear that if the glide path in the full-rudder sideslip is in fact steeper at 60 mph than at 50 mph, then the sink rate cannot be higher at 50 mph than at 60 mph.
Since the purpose of the full-rudder slip is to steepen the glide path and get the glider on the ground closer to the beginning of the runway, the glide angle is actually the parameter of greatest interest, but for whatever reason, in discussions between instructors and students I've heard the assertion phrased in terms of sink rate just as often as in terms of the actual glide path or glide angle.
Full disclosure: I have exactly 3 hours logged in a glider and have never specifically flown the 2-22 or 2-33.
That said, I do have some experience and understanding of slips, and the principles should be the same.To answer your question, I will draw from quotes taken from this article on Boldmethod.com.

In a forward slip, the amount of slip, and therefore the sink rate, is determined by the bank angle. The steeper the bank is, the steeper the descent.

However, there will be a limit to how much bank angle you can introduce before you can no longer hold a constant ground track.

In light airplanes, the steepness of forward slips and sideslips is limited by the amount of available rudder, or the rudder-limit. You may reach a point where full rudder is required to maintain heading.

If you are already at this point, with full rudder deflection, maximum allowable bank, and a constant ground track, there’s only one variable left - Pitch. Pitching down increases speed and increases descent rate.

As you lower the nose, airspeed and descent rate will both increase. The additional airspeed will give your rudder more effectiveness, allowing you to increase the slip. The opposite is true of raising the nose.

This means that you are absolutely correct. Properly configured, a higher airspeed should give you a higher descent rate.