But the percentage increase doesn't stack, does it? I thought it was then a 40% total increase.
It doesn't, but since your base Sneak level increases as you unlock those perks, it's still a substantial increase.
If you're at 100 Sneak and only have 2/5 of that perk, you're effectively at 125 Sneak. If you are at 5/5, you've got 140 Sneak.
I just think of it as a passive enchantment that would take a long time to make/find. I've never regretted going for it as it makes leveling Sneak a lot easier early in the game since you can be a bit less careful.