Are you currently experiencing performance problems? If so where is the bottleneck, is it with searching or indexing?
Also how many users are accessing the platform and how many searches are you running?
At a first look I would say another indexer wouldn't hurt, Splunk scales horizontally very well and adding additional indexers improves search performance due to mapReduce and also through reducing the IO load on the other indexers. Search heads are useful to add if you have larger numbers of users logging on and firing off searches but in this instance I would probably say an extra indexer (or two) but then as per my first comments, there are a lot of other factors 🙂