Hi , ok a few things first. , the ED80 on EQ5 is quite a capable set up and there are thousands of awesome AP,s out there taken with this set up.
Planetary AP can be done if you get a good 5x Barlow , TV springs to mind , you just need patience as I take it your set up in not in an observatory? ,permanently set up .
I don't think the 127mm Mak will be better than a Barlow,d ED80 , so save your money and put it into say an HEQ5 and keep the 80mm , great scopes these are.
Also with an HEQ5 you could mount a nice 150mmf5 newt for great deep sky shooting .
Not that the 127mm Mak is a bad scope. , far from it , it's kinda a step side ways from what you now have and dead money , you have to be really careful here as AP is really the. ' Slippery slope to empty pockets ' .
My 5c .
Brian.