Table.Buffer still doesn't help across different top-level query refreshes, which is what I meant by my original statement. Each refresh occurs in an isolated Microsoft.Mashup.*exe process, and Table.Buffer buffers the data in memory, so it's not really possible for buffering in one query's refresh to benefit another query.
As Andrey points out, however, if a query Q4 references a buffered query Q1 via two different dependent queries, Q1 will only buffer its data once. The fact that there are multiple dependent queries involved doesn't really matter. At the top level, this is just a refresh of a single query (Q4). If Q1 happened to be refreshing at the same time, the buffering would happen twice.
Hope this clarifies things a bit.
Ehren