The point of calling slow newts planet killers is that they give killer views of planets. A high F ratio is inherently better for high power views. A slow scope with excellent optics will give better views of planets than a fast scope with equivalent optics. This is a fact that owners of fast scopes need to accept.
I'm in the camp of thinking that a planet killer is any scope, regardless of f-ratio, type, or configuration, that has the capability of delivering a killer view of the planets. As others have said, realizing that view has more involved than just the telescope itself; managing thermal issues, seeing, collimation, viewer experience, etc. all factor into what is seen but reducing the variables to just the telescope itself, it could be anything of sufficient aperture. I do think that while smaller scopes can give excellent images of the planets for their size, if you don't have at least 6" of aperture and preferably 10" of aperture, you are unable to realize the resolution available on the best of the best seeing nights. Having said all that, my best views of Jupiter and Saturn ever were with a 18"F3.75 Starmaster. I think the additional aperture also helps a lot when using high magnification to see the smallest lowest contrast details. The additional light just helps make the difference even if the larger aperture is seeing limited.