Being very light they won't do anything against vibrations, but could still soften high pitched noises like drive motor or coil squealing. Any local hardware/builders store might have some thin bitumen tiles for cheap and such would be much better than duct tape at adding mass. Others use rolls of "roofers tape" for the same reason although personall I've never been able to find that for cheap, and some stuff made for long lasting outdoors use can be a bit foul on the chemical side.
Electrostatic is only a problem if the material gets very close to circuitry. Fasten any material well and check that the method of fastening does not start to give way after a couple of days because of the warm environment inside a case.
For what little I've seen so far the P280 wins on the pretty good drive mounting using one of the only good rubber/silicone style of buffers in the world of case manufacturing. The pre-applied thin sheet of plastic on the sides is probably doing at least a little good and leaves space for improvement with some foamy material. From pictures at least the plastic looks like it's sturdy enough to fasten additional material on top of it. The P280 is also a bit less expensive, although one might haver to block off the top vent with something less fancy than Corsairs magnetic covers. That they use some aluminium albeit probably just for the door doesn't come up as a bonus to me except for the looks. Any chance Al gets to resonate vibrations it takes.