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28" f/3.3 = 2347mm focal length.
With SIPS: 2699mm focal length.
Call it 2700mm.
25X/inch of aperture is 700X. This might be occasionally useful in good seeing in a 28". I have a friend who has used up to 1050X with his 28" and it did OK. But 700X is, though not usable all the time, not really the top magnification possible with a 28".
That's a 3.9mm focal length.
So if I were to suggest a planetary eyepiece that MIGHT get used a little more often, and which could double as a great eyepiece for planetary nebulae, globulars, and small galaxies, it might be a 4.5mm Delos.
Magnification: 600X. Only 21.4X/inch of aperture, so a reasonable high power that won't push the optics too far. And fairly practical in a 28" scope.
As for using a Delos as a planetary eyepiece, why not? It has good control of scattered light, it has pretty close to the world's best wavefront accuracy for a widefield, and it has no detectable tint. And it can double as a superb DSO eyepiece.
Someone who buys a 28" f/3.3 isn't looking for a planetary instrument anyway. It's a superb DSO scope that also can be used for planets and Moon and double stars.
And if, wonder of wonders, seeing is perfect, the 4.5mm can be put in a Barlow.................
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