The Poles considered Katyn is part of this in fact
Not quite what I'm aiming for.
Rather, the scenario is that Stalin decides to erase the Polish nation and exterminate the Polish people wholesale (similar to what Hitler's doing in the west). Something akin to a writ-large version of what
@Jan Olbracht suggested seems plausible, though the extermination component, I'm thinking of a multi-stage process in which the NKVD gradually rolls out the mass-killings much like their German counterparts.
For instance, a wave of multiple Katyn Massacres throughout eastern Poland to kill off the Polish leadership cadres, intelligentsia, and business elites — ridding them of figures the masses can rally behind or organize around. Beyond that, given that industrialized death camps aren't really Stalin's MO, he might just opt for another Holodomor to starve out the several million Poles left, while patrolling NKVD troops shoot survivors point-blank and relocates ethnic Russians to the land (who proceed to demolish the native landmarks and build atop the ruins of what was once Poland).
All in all, I'm fairly sure Hitler will be pleased — and even if he still hates Slavs, may just decide to let the USSR have a few more years to expend its own resources on doing what Hitler was planning to do to the Poles, anyway. Could buy Stalin time to build up the Soviet war machine, too, now that I think about it.