The problem with making a general purpose DB is you have to have general purpose defaults. The Cassandra defaults are "dont crash anywhere", not "be super fast and low latency". You can definitely do 5x better than the default with some basic jvm tuning.
That said: the IG folks certainly know how to tune JVMs. There are IG (and former IG, I saw rbranson post) folks in this thread that know how to tune the collectors, so assume that the 10x they see is beyond what you'd get from simple tuning.
Not sure you understand that there are different versions of Cassandra and I never mentioned that we used Datastax version. Using G1GC with default settings does not give you anything btw.
If it was as simple as tweaking a few GC settings to get 10x improvement pretty sure Datastax would've done it by now.