Thank you for contacting Toro and bringing your concern to our attention. The dealer has done everything right to this point and these items would typically solve the cutting issue you are experiencing. Engine rpm and travel speed of mower during cutting will have an effect of the type of cut you want to achieve. Engine RPM (3600rpm) should be checked to ensure the blade speed is correct. Height of cut can also play a factor. What HOC are you cutting at, and are you cutting at the same height with both mowers? Have you tried lowering your HOC to see if it makes a difference? If cutting tall wispy grass at high heights of cut you can loose vacuum and/or encounter blowout due to loss of the seal between the deck and grass.
Are you cutting the same properties with both mowers? Different properties / conditions can result in differences in quality of cut. Are you using Toro OEM blades, and are you using the same sail/lift blades on the GrandStand and Z450 mower? Toro provides different types of blades for different cutting conditions. There can be a big difference between high lift, medium lift and atomic blades. Time of year, grass type and the amount of grass you are cutting off can also require different blades. We are not sure of the current conditions you are facing so your dealer is likely to have the best recommendation that will work. These are just a few questions and thoughts that come to mind.
-The Toro Company