I've run the numbers. Statistically (excluding criticals) they are exactly the same.
Here's a quick sample. Let's say you are rolling two dice.
With focus, the chance they are both hits is 6/8 * 6/8 = 36/64 (that's an easy calculation - 4 sides are hits and 2 sides are focus - so 6 sides are good results)
Now, with target lock
First, the chance they are both hits to start = 4/8 * 4/8 = 16/64 = 1/4
But, what if the first roll doesn't hit. There are two options,
The first roll is all misses, then the reroll is all hits = 4/8 * 4/8 (misses) * 4/8 * 4/8 = 1/16
But, if the first roll is one hit and one miss, or one miss and one hit, then 4/8 (hit) * 4/8 (miss) * 4/8 (reroll hits) 1/8 x 2 = 1/4
Thus the total for target lock is 1/4 + 1/16 + 1/4 = 4/16 + 1/16 + 4/16 = 9/16 = 36/64
Exactly the same. Here are ALL the stats….
Hard to follow, I know. But it works. No matter how many dice you roll. And I calculated all the options. Listed here