There is certainly value in functional testing on a limited scale, but you are nearly always giving something up as you scale back from a true production parallel environment. To answer your question, one would need more details about the application. What are the expected load patterns? How contentious will the ultimate production envirnment be? Do you have any potential bottlenecks (such as a database) that wouldn't hint at a problem until you scale-up? What are the consequences on an outage?
Based on the answers to these questions, it may even make more sense to test on a limited environment, as it may be easier to analyze what is happening and focus on specific areas (maybe your last bug fixes!). Also, a full production parallel doesn't mean you are testing production as the traffic patterns of a real running system (connected to many other systems, both internal and external) are very difficult to match with test harnesses.
GemFire--The Enterprise Data Fabric