I don't have an Astro, but I did put a 5.5" plate on my 17' fish-n-ski Procraft. I don't have anything to compare it to, as installed it on the boat as part of my restoration. The boat was trashed when I bought it, so I never ran it without a plate. But, in my opinion it helps me out. You can fine tune the motor height based on your style.I am getting low 50's out of my heavy fish-n-ski with a 130 hp. Per Pat on the setups board, that is all I can expect.
Keep your eyes open for a used one. I ended up putting a 5.5" CMC on mine and I haven't had any problems with it. I don't see how a plate would hurt you.
I think you can probably go to about a 6" plate, before you will have to start looking at replacing cables and such.
Modified '92 Procraft 170 Combo/'97 Johnson 130