I don't really have enough data points on Sydney seeing, but from what other folk say it's not ideal
but like you say, there are exceptions to the general rule. I think my point is that you might be able to get use out of a 6 but infrequently, whereas you should get much more use out of an 8, so you'd get more of your money's worth out of it.
Personally, I prefer to see the planets at lower magnifications (165-200x) where the object might be smaller but generally more crisp and with more contrast, rather than bigger and blurry, but that's me
Regarding the quality eyepieces, it's not just about the design and manufacturing tolerances that makes them better...it's the quality of glass used, the coatings on the surfaces, the baffling and blackening internally to reduce flares and other reflections... the example I gave of the TV before, it's just a plossl ultimately (and plossls can be great so this is not a negative!) but the TV is one of the better made, if not the best at reasonable cost. Quality eyepieces benefit every telescope, a slow telescope just makes them appear less awful, but the detriment to the image is still being effected.
Have a look out for a Japanese made ortho...simple design, little glass, well made and generally inexpensive for decent brands (Baader, Astro Hutech/UO)...great value for money specifically for planets and lunar. Only downside is the narrow FOV