I guess it depends what defines a planet killer?
I would think a 20" Ostahowski f/3.4 would be a fine instrument for planetary observation once cooled and well collimated... 20" of well figured aperture would pretty much kill anything you throw at it.
I have to admit that even though I have a 16" f/4.5 scope with an Ostahowski mirror, I'm still working on an 8" f/~8.7 dob specifically for planetary use. The 8" f/8.7 I have isn't necessarily because I think it would be better than my 16" for planetary...I'm building it just because I can, and out of sheer curiosity about how long focal ratio reflectors operate (all of my experience is with F/6 and shorter reflectors). I'm using an old thick Pyrex mirror that I still need to get tested and maybe refigured, but I assume it'll do well in the planetary arena.
I have a vintage C8 SCT which in theory should be great for planetary, but I have mixed feelings about how SCTs perform in relation to refractors and the big aperture of reflectors.
I have an f/4.5 16” Ostahowski mirror, and also an 8” f/7 by Steve Lee. Both appear excellent mirrors but the 16” slam dunks the 8” on everything. As it should. Every time I think of building a planet killer...like, say, a 10” f/7 or whatever, I remind myself I already have one.