I have a 7 agility, and currently 3 points in the sneak perk.
You can get a silencer on a pipe gun at gun nut 2 btw. I have a hunting rifle without a silencer, and a pipe rifle with one that hits for a fair bit less damage. Once I get to gun nut 3 level, I will probably retire the pipe, despite having so much ammo for it compared to the .308.
For armor, I have modded just recently muffled on both my leg pieces.
There are a few things to note about sneaking.
1-if you are moving, you are less sneaky than standing still.
2-Line of site and proximity are both about equally important.
3-The brackets are important, watch the brackets.
4-Some encounters seem to be hard coded to be more perceptive.
5-Ghouls hate you.
6-Shoot the dogs first.
When you are hidden, you will see the brackets [] moving closer and further from the word hidden. when they are on top of the word, you are the most hidden. When you move or get closer, the brackets move away, if they get far enough away, you will go into caution. Time spent in caution will go back to hidden. Making noise also generally moves straight to caution. If you see the brackets getting far, waiting a few seconds in a good piece of cover until the brackets close again will help you continue to move unseen.
When you move into caution, enemys will roam a bit further than normal, especially where they heard whatever put them in caution. So if you snipe someone, move. They will often come look at the source of the gunshot, but being in a bush 10 feet away is far safer than staying in the same bush.
If you shot and did not kill someone, they tend to be more persistant. It is often worth taking another shot or two on someone you alerted before retreating to wait out the caution, but only if you are sure you can drop him before searchers get to your position. This is especially relevant with dogs involved. if they get anywhere near you, you will go to danger, and they are quick.
If you do get to danger, you are probably best served swapping the sniper rifle out for a combat rifle or shotgun and finding a defensable position. Danger will drop to caution if you can run back into far enough away cover, but the enemys will tend to home in on you a lot further than in caution.
If you are indoors, often enemys have locations that they will not path through, so you can use these as spots to retreat and wait out caution or even danger sometimes. But also indoors, enemys can hear you through walls, and run around to attack you if you arent careful.
Ghouls will come directly at you even in caution, and act a lot like dogs, but not quite as fast. They also tend to bring all thier friends in the neighborhood.
Overall, I feel stealth is a lot of fun in this game, but far from the invisibility that other bethesda games have allowed it to be. And the sneak perks are worth their weight in gold if only for ignoring mines and tripwires.