There is still a fact you are not grasping. Production, or in the marxist language 'ability' is limited. You only have a specific quantity of ability in the population, that amount is 100%. Needs can be infinite. So if you take away from that 100% of ability to fill a social need, the social need doesn't have to align with a personal need, and often times doesn't.
Measure is pretty simple in X if it is considered direct exchange, as the model I mentioned in the past. 100% value exchanged for 100%.
Y gets complicated, from listening to you describe your model for many weeks, whatever resource production, there will be approx. 8% for collective means of production upkeep plus a collective distribution scheme. 8% for the charity for bums. 8% for that free stuff and lambos the vanguards keep selling the model on.
Therefore the workers will not receive 100% of their time and effort returned, or will have to work extra to achieve the same amount of resources that X produces(especially if they don't care for bums or lambos).
X gets the productive workers all they want.
You are also assuming 'haves' in your ' dance to the tune' remark. In the direct exchange model there isn't a requirement of 'haves' existing. Your still assuming X will have social constructs. The X model will still strip all the productive workers even without social constructs.
Also requiring the workers to produce the 24% for social programs makes them less free to allocate those resources for something else.
That opens up problems in social objectivity of what a social system should produce, but i will save that topic for another time.