We actually haven't been running against any limits here. One thing to keep in mind is that postgres remote-fetch operations aren't tuple-at-a-time, so this shouldn't be a bottleneck for our multi-node operations.
An example would be TPC-H Q1, which is a little weak on the group by cardinality, but is good for testing raw aggregation performance.
We're actually currently focused on query optimization for our multi-node product, but we don't have any numbers we're currently ready to share.