Hi Dave,
It could be the FL (300mm with this lens) used in combination with the Marumi +5 Acromat.
I have both of these, and just tried it. I get similar results at 300mm with this lens combo, but at shorter FLs with the same lens, the Marumi +5 shoots considerably sharper, so it's possibly not the Marumi's fault.
I'd previously used the Marumi on other lenses (mostly with 50-100mm lenses adapted to the Q) with no problem, so I was curious about your post. I don't recall getting any recommendations with this achromat, but in the past, these diopters commonly were optimized for specific lenses and/or FL ranges. I'm thinking that this one is not optimized for use at 300mm.
Here's a site that lists a lot of Achromatic diopters (but not the Marumis) and shows that the Raynox DCR 150 and 250 are optimized for use at 300mm.
Fuzzcraft.com | A list of achromatic close-up lenses | Photography, audio and light DIY projects
I've just confirmed this somewhat by trying it on my FA* 300/4.5 (so the lens is not at fault), and I got similar bad results. However, with my DA 50-200 at 200mm, the Marumi shot fine. It looks like this diopter is optimized for lenses up to @ 200mm.
Scott