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.
The resolution and contrast are strong functions of aperture, weak functions of focal ratio. What this means is for a given focal length (and OTA length) a well fast scope will be a better planetary scope than a well made slow scope, it has the aperture advantage.
In the old days, when scopes were relatively small, my 12.5 inch F/6 Meade RG was a monster scope when it was manufacturers, and the skills to make fast mirrors were lacking, slow scopes were considered planet killers.
That was 40 years ago.
Today, a 12 inch is a medium size scope and people are using much larger scopes for viewing the planets. I have to think that Jeff Morgan's 16 inch F/7 was a slightly better planetary scope than my 16 inch F/4.4, smaller secondary, that sort of thing.
The problem is that 112 inch focal length. That takes a serious ladder. Jeff wrestled with that for a number of years. He worked on the low rider concept and a periscope but I believe I recently saw the mirror for sale on Astromart. my 16 inch is still alive and going strong.. No ladder needed.
So in terms of a planet killer, the practical aspects have to be considered. It is about the view and the viewing experience. A 12 inch F/6 has a focal length of 72 inches, a 16 inch F/4.5 has a focal length of 72 inches. Both will be flat foot scopes for a 6 footer. Certainly the 16 inch F/4.5 has more potential and if properly made can realize that potential.
So what fans of slower scopes have to realize that it's different world today that it was 40 years ago. Mirrors are thinner, better materials, quartz instead of pyrex, properly supported and thermally equilibrated. They cool quickly, they use interferometers to test and figure the mirrors.
This is a comment made by Roland Christen:
"...How about a custom 17" Cassegrain with precision quartz optics, small secondary, 100% US made? Optics by Lockwood and AP. We made a large Cassegrain with Lockwood mirror, which I showed at NEAF this spring. The mirror has the smoothest, most accurate curve on it that I have ever seen. The scope is light enough to be placed on a mount by one person."
-Roland Christen, Astrophysics, 16" f/3.5 lightweighted quartz primary mirror by LCO, other optics and telescope by Astrophysics"
When Roland says the mirror has the smoothest, most accurate curve he has ever seen, not much can top that.
Planetary is about aperture, seeing, optical quality and care. It's not about focal ratio.