Never got around to having the P01 milled for a red dot.
I have had two P07s and one P09 milled for red dots.
I shot the .40 P07 and 9MM P09 a good bit before having the work done. I don't remember any difference in recoil. There were zero functioning issues (as in the weight of the red dot minus the weight of the removed slide steel causing functioning issues).
I use kydex holsters. I ordered one for the suppressor ready P07 model so the taller back up sights (front sight) was no issue with that holster. I did have to trim some kydex and then heat it up with a heat gun and spread/shape it some to fit the red dot. These days you can get holsters set up for red dot pistols.
As for shooting them and having issues with sights (red dot vs. iron sights), no issues there either.
Many people report initial issues when trying to get used to the red dot vs. iron sights.
Iron sights = line up the front and back sights, keep them aligned while putting the front sight where you want the bullet to hit on the target. Easier with more practice.
Red dot = see the red dot in the window of the sight and put it on the target. The dot doesn't even have to be centered when you're up close (inside 15 yds, maybe farther - I've not tried it farther out). Again, easier with more practice.
I'm 65. I can no longer see that front sight without my glasses, so red dots are the way to go. Without my glasses the iron sights are useless. Without my glasses the red dot is blurry, but I can put a blurry dot on the target all day long.
Do be aware that many places mill the slide for a specific red dot (there are usually some other brands/models that will fit the same sized spot). Some Holosons and some Trijicon pistol red dots are that way. Not all those two models have the same sized "footprint" and you can't go from one brand to another unless it's one with the same "footprint" as the sight you're replacing.
You'd have to do some research to find out what brands/models will fit the same spot/screw hole spacing, etc. as the Holoson you are looking at in case you decide to change to another model/brand some day.