So, two points. Gym mat may not always work under a sub, and I am not a fan.

But your thought is spot on.
Cos, in the same vein, however, i found a huge difference years ago in controlling the highs in the room by putting bathroom floor mats right at the first floor reflection points of my LCRs. Made a world of difference.
I then paid a few hundreds more and put a few fluffy area rugs in the same spots, and it got better.
Then got greedy, and spent a few thousands on a carpet. And i learnt what diminishing returns mean.
I really couldnt discern any difference between the area rugs and the carpet.
Lesson learnt.
Basically, from 50 buck product to 500 buck product, huuuge difference.
From 500 bucks to 8000 rupee carpet, feeling shitty. They are basically doing same job.
Which brings me to this.
i drew the line at obviously bad sounding things and it sounds great.
These days, i dont spend sleepless nights wondering if i can juice up my room to gimme me more by spending thousands! I just enjoy the experience knowing my room is not perfect, but it is well damn NEAR PERFECT TO MY EARS IN THE SPACE I HAVE.
I might still upgrade, but only when my current components die. Till then i just am happy to do small tweaks within my set up.
Lovely question/thought, again. Am glad you put it out here. And am really enjoying the replies.
Do let us know how the gym mat goes.