In a similar situation we just adjust the GC and started to use G1GC which resulted in similar numbers.
If it was as simple as tweaking a few GC settings to get 10x improvement pretty sure Datastax would've done it by now.
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.