I would suggest setting up a hot/warm architecture where you index data into the i3.2xlarge instances and then move indices off to nodes backed by gp2 EBS once they are no longer written to after a few days. Large gp2 EBS volumes are considerably slower than local SSGs but get a decent amount of IOPS even without provisioned IOPS and should be able to handle querying quite well. Indexing is however very I/O intensive and probably best left to the current nodes (although you may get away with fewer than you have now).
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