Your current basic dish and the HD Ka/Ku dish will point in the same direction, but the AT9/AU9 will need to be tilted and have additional 20 degrees of clearance on the right side (101 to 119 or 99 to 119 to be precise). Also, the sats don't sit in the same elevation, so that opening in the trees could present a problem.
The only way to determine signal strength is to test a dish with a signal meter at the mounting point; being a roof mounted dish, this would be difficult with the large and heavy Ka/KU.
I recommend using a triple set (3LNB) dish for signal testing (101 to 119) using the same mast you have now before considering the 5 LNB dish with a heavier mount. Also, the slimline should be installed with 1 or 2 extension arms (monopoles) on roofs to keep it in position.