You're getting into a whole new world when you start adding lenses to your site. I personally hunt with a 2X lens in my HHA and a 6X lens in my 3D scope. The lense in my HHA is more for clarity, not magnification, due to my glasses.
One thing to remember, adding magnification to your site is like looking through binoculars- the object at a distance (the target) becomes larger and more defined. The object right in front of your face (your hand in front of a bino and the pin in the sight) becomes blurred, distorted, and can even disappear. If you stay with a 1X or 2X lens, you probably can get away without any other changes, but once you go to 4x,6x, or 8x- you are going to need to add a clarifier peep. This usually requires something like a superball peep set, where you can install (via threads) various strength clarifiers (#1 yellow, #2 green, or #3 red) to clear up your pins enough to see them. You also need to pick clarifiers that have the correct apertures for what you are doing, like 1/8", 3/32, etc.
Don't panic or quit now, the best advice is to go to a pro shop that sells a lot of target archery supplies. There is no hard rules on what to get- you have to try them in combination-because individual vision deficiencies will dictate what works.
Only con's I know by adding magnification--1) you cannot shoot in hunter classes, you must move to the open classes (longer shots up to 50 yds), rain (like looking through a fish bowl when it get wet), and fog on the lens- for obvious reasons.
I shoot a 4x for indoor & for 3-D. I have not used it for hunting. I have found for indoor I need to put a clarifier in. For 3-D, I don't need it. Alot has to do with lighting indoors. This is why I haven't used it for hunting, as the light dims I have trouble with needing the clarifier.
You really need to experiment with powers & clarifiers to see what works. I have a friend that uses a 4X for hunting with no problems.
They are a great add-on but do have thier pros & cons. I used to shoot a 6x for 3D, I had to shoot a clarifier because it was just to blurry. I loved it!! I decided a few weeks before hunting season that I was not going to hunt with it for possibilities of fog, glare & close shots, so I took it out & unscrewed the clarifier from my peep. Went out to do some shooting, when I drew back & took aim for the first time at 20 yds it felt like I was trying shoot a 60 yd target. The lens will definitely spoil you. I ended up not shooting for 2 weeks to kinda forget what I was used to seeing with the lens. If I were to do it again I would probably do a 2x and leave it in all the time, if there would be a chance of fogging when hunting, I'd just remove it. I don't think you'd have near the transition as I did going from 6x to nothing. They are simple to install, basically a lens, o-ring, cap & 2 screws.