Low impedance pickups with metal covers
 

Let's say you wanted to build a low impedance thunderbird pickup. Would the metal cover smooth out the highs, compared to a normal high impedance pickup?
And yes, I'm planning on using a preamp, possibly an East filter based.

You cannot generalize tonalities by comparing one side of the impedance range to the other. It means nothing without accounting for other factors, and specifics. In any case, metal covers tend to darken things a bit, due to their interaction with the magnetic field. If this is desirable, go for it, but if it is not, opt for a chromed-plastic cover. In general, a low impedance winding will shift the resonant peak up, resulting in a flatter frequency response. Low impedance pickups are a rarity, however, as they tend to have a low output. If you don't want to build an active pickup with an internal gain-boosting preamp, a transformer may be used.