Interesting, for $50 buck might be worth the investment. Good feature is that you just slide them on (need powder?) and easy to take with you. Probably would have to include some dead stops (starts off the chest...without the slingshot) to keep full strength balance. I had to look the slingshot up, new to me.
Guy's have used bumper plates, when doing floor presses, for that super rebound affect. Or even wooden planks, for rebounds.
Bumper plates are exceptional for DL's and cleans also. As are the old style Hooper DL's, which no one seems to know about these days. Any of these versions; slingshots, bumper plates or Hooper's greatly help with using weight well beyond the normal abilities. Or what they may thing is normal. Truth is, no one really knows are strong they really are, until tested.
Don't know if Donny posted this in the PL'ing/Strongman thread, might get some better answers there. Good luck.