You got me chuckle
Mostly at myself and my clumsy sentence on sharpness. You are absolutely right, whatever you put in front of the lens, it will
effect on quality. Mostly negligible and practically unnoticeable, unless you use a Coke bottle, cheap knockoffs, uncoated filters, etc.
But that is not what I had in mind. I meant that sharpness is overrated, that we photograph for a myriad other reasons, the least
of which should be sharpness. Unless, of course, you are an optical engineer and your employer is DxO or a similar. We do not photograph to demonstrate sharpness (with one exception: proud owners of the new Sigma DP2 Merrill
), thus my concern with the OP sharpness angst.
That's a valid concern, but I have another concern to trump yours ...
ND grads produce an artificial look on all subjects that don't have straight horizon outlines. I can understand the requirement to use Grads with film, but with digital capture they are often a worse choice compared to e.g. bracketed exposures and some postprocessing (or proper tonemapping of a high dynamic range image capture).
More and more these days we hear statements like: "I do not want to use filters, or ISO over 400, or f/stop beyond diffraction-optimal, or..., because I might lose sharpness, increase noise, etc." You should shoot with whatever is necessary to get that shot, and worry about the rest later (or not).
Dogmas are never sensible. However, one can only consciously decide to break a rule when one understands that rule (and its limits). More often, ignorance is at the basis of poor decisions.
What's the point of getting a perfectly sharp shot if you end up with blown skies or too-dense shadows (one of the situations where a GND filter might save the day)?
No point at all, unless there is a better alternative. That's what I try to promote, master your equipment with good technique, instead of having your equipment master you. I hate it when poor technique distracts from enjoying an otherwise great image.