Generally (IMO) uncovered pick-ups have a bit more open tone.
It's like they respond slightly more compared to covered pick-ups.
I've tested both Fralins and TomHolmes with covers, and then taken the covers off.
Anyway, I use several covered pick-ups and they sound great (of course).
I guess Tim has done something to add up for the loss of treble in covered pups. My HoldyDivers are with aged covers, and sounds awesome.
I would not change a bit on these.
T.