To gain the scalability/performance of a k/v store while having the capability to do complex queries (e.g. joins, range queries)

I believe you would be much better off with a DSL that mapped to a set of fast operations on a k/v store. SQL databases are heavily heavily optimized, and far more performant than k/v stores for most non-trivial queries. k/v stores don't somehow give magical performance. What they do is limit you to a small set of primitives that helps with scalability and maps more naturally for applications that are document-oriented and don't slice and dice data as much.