The former is still struggling to get counters done right, the latter promises not lock table for every write operation... in next release, of course.
OK, here is the link to the tech paper:
It contains everything except raw benchmark performance data, but the most anecdotal statement can be found at the end of the document:
Because in memory tables are stored on the JVM heap, the total SIZE_LIMIT_IN_MB for an in memory table should currently be limited to 1GB per node, with amount also being dependent on the heap size and the need to leave room for normal Cassandra activity.
Cassandra, you say? :)