While there is no maximum allowable/usable JVM heap size, there is a point where the task of managing the threads involved in larger heaps and the associated Garbage Collection outweighs the performance benefits of having the extra threads.
You could set the maximum heap size higher, however, our performance test results indicated that there was no further gain to be made in raising the size past 6GB and instead it was better to add a new proxy if further capacity is required.
Remember that the recommendation in the Best practice guide is for 6 GB RAM on a VM with 8 GB total RAM AND 8 CPUs (for example, 2 quad cores). The CAM proxy doesn't retain huge amounts of information in memory over time and each request/response is handled and the memory freed very quickly and performance is bound as much by processing throughput as total memory available.
Note: It is possible that raising the heap size and not increasing the CPU allocation accordingly could cause a performance degradation.