What is the write performance of Cassandra, HBase, MongoDB or Elastic Search
Cassandra is pretty fast indeed.
Voldemort is very fast too. I know at least one massive website that chose it over all other options.
But, really, 50K/sec is nothing special. I did it with MySQL on one machine.
Note, however, that besides how quickly you can write data, you should be also interested in what can you with that data later. Otherwise you can just pipe all your writes to /dev/null. It will be insanely fast.
Also, any "benchmark" you find in the internet is useless. It either uses data with certain characteristics or synthesized data. And this can make all the difference. Nobody will do a benchmark of your scenarios, using your data, except for you.
You might be interested in Netflix's benchmark of Cassandra up to one million writes per second on EC2: http://techblog.netflix.com/2011/11/benchmarking-cassandra-scalability-on.html