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Whether you use an aperture stop or use the yellow/ green filter to filter out the defocussed halo of blu/violet light at the green focus - you are simply throwing light away.
I've owned one of the achros for a number of years . I find the highest magnification I can use for deep sky where the image looks acceptable is about 18mm . Higher magnification than that and you cant escape the stars all looking like slightly fuzzy yellow balls . At the same time the view is pretty nice with 32 to 24mm eyepiece and large and bright clusters and nebulae can look good.
I think its just a case of not having too high expectations and being occasionally pleasantly surprised . Saturn being fairly yellow and monochromatic can look pretty good at high power but medium to high power on the Moon is really marred by secondary spectrum.
As previous poster said everyones sensitivity to chromatic aberration varies , but I guess as a Newtonian user for years I don't adjust easily to it . I think its well worth getting hold of a second hand 100 to 120 mmm ED , than a 150 mm achro.
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