You're missing the point. Vibration isolators have nothing to do with making you comfortable standing next to a running machine. You can't declare them to all be equivalent by saying you can't feel any difference.
Vibration isolators are required to prevent vibration stress cracking in the receiver. In order to be effective, they have to have to be able to dampen those cyclic stresses as the machine is running. Using something that's as hard as the floor or the steel is useless, regardless of whether you can "feel" any difference or not.
For the OP, there's another easily-acquired solution available at Home Depot, though probably not the cheapest. They sell sets of isolators for front-loading laundry appliances that are very heavy duty and well-suited to the weight of the compressor you have. I used four of them on an 80 gallon Champion. Note the bolts are NOT into the floor, they just hold the isolator to the compressor foot, and the isolator sits freely on the floor.
Um, the word used, by me and other was HEAR

Yes, sound is vibration, but that has nothing to do with what was under discussion.
Nobody in our little test could 'feel' the floor vibrate from any of the compressors, at any time.
We were doing sound checks to see what might work better as all of us have compressors and all of us hate the sound.
As to the value of difference to the compressor from isolation mounting vs. hard mounting, I will defer to the manufactures.
Who suggest hard mounting. At least on the six or eight brands I've worked on.
Now mind you, I much prefer 'sort of soft' mounting myself, but call your machines builder and they tell you it doesn't matter.
On the machine I'm using, I have hard plastic (mostly to raise it) hard rubber, rubber washers between the bolts, and rarest of all, the compressor is mounted on a hard rubber pad.
This goes between the compressor and the metal pad on top of the tank. I tried it, and found a small difference in sound, but a BIG difference in being able to change the oil without giving the tank an oil bath.
The isolators value is just not under this discussion, or at least, the part I addressed.