This last fall I bought the Impact Gel XL Lite pad (1/2") to use with other pads or blankets depending on what horse I rode.
I was worried about the breathability of the pad and went with that particular one because it was supposed to more breathable than their original pads. Not sure if I could tell the difference since I am a stickler for getting off, uncinching and lifting my saddle up to let their back breathe anyhow.
I quit using it because I was getting rub marks over the loins of my horses that I have never had an issue with before, same saddle, nothing changed except for the pad. Can't be certain it was the pad but like I said it was the only thing that was different.
Mine did freeze this winter and that is why in another thread on here I suggested if you do get one maybe keep it inside at night. Mine was in the barn, not in a tack room per say and it froze solid.
Don't have any suggestions for a mutton withered horse but I wouldn't spend the money on an Impact Gel in my opinion. I went back to my old wool pads. If you insist on trying one maybe borrow one or buy a used one so in the event it doesn't work you aren't out a whole lot of money and you could re-sell for close to the money you bought it for.
Good Luck :)
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.