Monday April 9, 2018:
I was noticing some BBS discussion about installing new Moss seat cushions, and some related questions about solid foam rubber or cutting out core holes to soften and lower the seat. Having two day's experience sitting on new Moss complete seats (which seemed to be too high and too stiff), and a friend with new cushions in process of installation, it was a timely opportunity to do a little experimental investigation. So I called my friend and made arrangements to pick up a 1-1/4 inch hole saw, get courageous and maybe carve up one of his new seat cushions.
One set base that had just rebuilt had the base board upside down, ending up with two right hand seats. So we took a few minutes to disassemble and reassemble it again to be a left hand seat.
The next few pictures should answer a BBS question, that there is nothing wrong with the ramp angle on the bottom of the new Moss seat cushion. This would be a problem with the Scarborough Faire cushion.

First we assembled a new solid foam rubber cushion from Moss onto the seat base, put that in the car, and sat on it for reference. It seemed softer than the new complete seats in my car, so we put the leather slip cover over it and tried sitting again. Big difference. This is when it felt like sitting on a rubber ball that would bounce but was reluctant to compress. My take on this is that the foam rubber is not breathing, but compressing the air that is trapped inside.
So we decided to try coring one of the cushions for comparison. Since the passenger side will be used less, we did that one first. If it came out too soft, then maybe not do the driver side, and some years later they might settle to similar height. The hole saw made very quick work of this, boring 60 holes in a matter of minutes. Like cutting air, bore the hole, then poke fingers in all the way to the bottom and pull out to core plug. Holes in the thickest part of the cushion are 2-inches deep. Holes in thinner parts are 1-1/4inches deep. This leaves at least 1-inch thick foam at the top of the cushion. Holes are 1-1/4 inch diameter on 2-inch center distance, 8x8 grid minus the four corners. We decided not to attack the 9th row at the rear edge, as it will be under the backrest where no one would sit on it anyway.

Then reassemble the two cushions, and get back to the sits testing. Big difference. Sitting on the cored cushion results in immediate squash of about an inch, and no bouncy feeling. I'm sure this is mostly due to air breathing out instantly. If you intentionally hop up and down on it, the cushion changes thickness almost instantly, like a good shock absorber should (so to speak). No more hard bouncy ball feeling.

The rest I think is up to the passengers' preference for ride height. In this case my friend elected not to core the driver side seat cushion, because he is shorter stature and likes the higher perspective. Do give him credit for allowing me to carve up his passenger side seat cushion (for benefit of the MG community at large). People of taller stature in particular may like to know that the seating height can be lowered at least an inch just by coring the bottom of the cushion (as original). If you like the softer feel but do not want to lower it so much, then maybe use smaller core holes for good breathing but retaining more material to retain more support (less squash). This is also a good time to pop a few breather holes in the front ramp board if your replacement seat base does not have them. See construction article INT-202 for those dimensions.

Thursday April 12, 2018:
We dashed over to our friend's place to pick up the new electric stapler and finish reassembling his seat bottoms with the new cushions.

It is not a coincidence that Dave's vintage cover with the nice patina is similar profile to my new cover, as they both have the new Moss Motors cushion inside. Notice the high front end and slight dip in the center (even with no load on it).
We took turns sitting on the cushions with new solid foam and new cored foam. Dave likes the solid foam, even though it is more firm and bouncy, because he is shorter stature and likes the higher sitting position. Navigtor likes the solid cushion (by comparison), saying he sinks through the cored cusion, like hitting bottom (but do consider that he weighs at least 250 pounds). I like the softer cored cushion, but I also like the higher sitting position, so if I was to core my seat cusion I would have to put the riser blocks back under the slide rails (because I'm a shortie too). There can of course be variations in between if you use a smaller hole saw or fewer core holes.