The description of optimized servos does not tell you, how much it reduces the AP cost. By the search over the internet, many people observe that both legs gives you -30 percent AP reduction.

However, it was a measurement almost 10 years old. I did a large test for both T-51 and T-60 just now. Optimized servos on both legs actually reduce AP cost of sprint by 40 percent. My tests was about to run on END = 15 as long as I use all my AP with non-upgraded legs. Then I run the very same distance and on both I got 60 percent (I double check all my stats that endurance and agility was on the same value for each run.

But that was not my only observation. There is also one another observation I did.

Non upgraded T-60 - 100 percent cost
Both legS optimized servos T-60 - 60 percent cost
Non upgraded T-60 with Atom Cats painting - 82 percent cost
Both legS optimized servos T-60 with Atom Cats painting - 42 percent cost

So as you can see Atom Cats painting which suppose to give you 18 percent reduction on all pieces, is stacking additively with Optimized servos which makes the combination of these two upgrades quite good.