It will be very helpful to have a non-product flag in G1 that artificially allocates regions in the heap to increase the rate at which the used space in the heap grows. This will increase the frequency of marking cycles as well as cleanup opererations and will allow us to stress test that code when we need to.
We could use specially written tests which fill up the heap quickly to achieve the same effect. But this feature will be generally more helpful as it will do the same while running any application in order to generate different loads on the marking cycle (mutation patterns, object graph shapes, etc.) and stress it in different ways.