there is a point of diminishing gains and increased sides that will vary with different individuals. I've know guys who grew like a weed at 300mg/week, they certainly would not have grown 10 times as much off of 3000mg/week, however the sides at that dose would likely have been tremendous.
personally test does not pack on mass like a 19nor, but I love the way I feel on test, so I cycle them together for the advantages of both.