You should create your own datastore for this kind of behaviour but I think will not solve in deep the problem if you really have it.
Consider 1000 concurrent users with write grants do not it means you have at the same time 1000 documents being written at the same time. Here the question is not the number of concurrent users, here the question is the number of concurrent writing actions, and if you want to test this environment you need only a concurrent robot what execute writing calls to the OpenKM API.
In the end, you will have a number that will indicate how many documents can be written during a minute or during a second if you have a good performance. You are only considering the datastore ... but I will be more worried about database than datastore. Are several ways to optimize OpenKM as a data store, for example, remove security and operate always with a single superuser, set Lucene search engine async, etc...