How much data is needed to show MySQL Cluster's performance scaling vs. InnoDB
MySQL Cluster stores indexed columns in memory, so the largest practical data size for an in-memory cluster is around 3GB.  If you store tables with non-indexed columns on disk, then you can scale the dataset further

As MySQL Cluster automatically shards your tables across multiple nodes, with each node being a master. This gives very high performance for OLTP workloads, especially those that are write-intensive.  To get the best levels of performance, you need to introduce load to the cluster, ie configure multiple API nodes, with each node configured with multiple threads to access the Cluster. 

The latest 7.2GA release of MySQL Cluster includes new functionality called Adaptive Query Localization, which serves to push JOIN queries down to the data nodes, reducing network hops and therefore latency.

To get the best handle on performance, there is a Performance Guide published as follows (note, registration is required):
http://www.mysql.com/why-mysql/white-papers/guide-to-optimizing-performance-of-the-mysql-cluster/