I have the Royal Rider irons and am finding that I can't use the cheese graters as my feet are not comfortable. After years of riding the balls of my feet have become sensitive to them. The rubber ones are comfortable, but I would like more grip, especially in the rain as I find that I cannot rely on pushing all my weight in my heals without my feet slipping out.
You probably could find something to file/sand them down a bit, but you'd probably be better off going with a different type of stirrup pad.
I have the Super Comfort Pads, liked in the post above, on my dressage saddle, and do find them comfortable and grippier than regular rubber pads, and actually like them better than the cheese grater knockoffs I had on it before. On my jumping saddle I use ProJump pads, which screw in place, and are great. IMO, they've got more grip than the Super Comforts, but aren't sharp like the cheese graters. And the pair I have is almost 10 years old, and shows almost no signs of wear.
I suppose you could tap them down a bit with a small hammer but I'd probably just use a super comfort pad, like the one a previous poster linked. I've got some and they're fantastic in the rain and for every day. They also don't scratch my saddle even when run up.
Trying a life outside of FEI tents and hotel rooms.