OK, one thing is that it boasts "automatic distance selection (ACK Timing)" so my comments about lightspeed are somewhat less relevant.
The spec for the 500 also calls for a 60 lb wind loading force at 60 MPH. Your 6061 aluminum pipe (nominal 1.25" Sch 80) yields at 24000 lbs, you have a Section Modulus of 0.291 cuin. Bending stress at the bottom of the 18' pole is (60 lb x 216 in) / .291 = 44,536 psi which is nearly double the pipe's strength - it'll bend long before 60 MPH is seen.
Source (pdf)
And I still thing you're underestimating the effects of tree sway. The 3 dB beamwidth is shown as about 5 degrees, which is pretty tight (but typical for a dish, or a Yagi with comparable gain), wobble will kill your performance.
I don't disagree you need to get above the trees.