Winn grips are affected by DEET insect repellent, as are most plastic and rubber objects. They are used on muskie and salt water rods, so they are in fact pretty durable. As for sensitivity, since they are pretty light (lighter than cork) they should be pretty sensitive, but you know sensitivity, no objective test for it. The inner material is EVA, and the skin is molded to it, so it cannot be replaced. However, it would not be that hard to cut off and replace a rear grip. A front grip is less likely to get worn out, and getting a new one one would most liklely require taking the guides off, so that's not a bit easy.
Winn offers Winn wraps, a tape like bike handlebar tape and tennis grip tape that is the same "sticky" material as the surface of a Winn Griip, and they are easy to use. In fact if I had to replace any kind of grip I would use these. If you want to find out how Winn grips feel in action, about $10 will buy a wrap that you can put onto any other grip material.
Winn grips are a core of EVA covered permanently with the "magic" Winn material. The cores are reamed to fit the blank, then glued on with epoxy or double sided tape like golf grip replacements.
Then there is the Winn wrap which is explained in another post here.
I should clarify what I meant by replacing a grip with the wrap-it works for deteriorated or chipped or other nasty grips, but it is a tape and as such it won't replace a structurally compromised grip. For that a remove and replace is required.
The wrap will work for deteriorated but not structurally compromised grips. It's just a tape. To fix a structurally damaged grip requires a remove and replace.