Considering how long it takes for a proton torpedo to lock on a target, the torpedo should not be evaded once lock on is complete. As for concussion missile with second longest lock on (if I recall correctly), evasion should not be successful once the missile is halfway towards the target.

The evasion stat does not affect secondary weapons outside of railguns and rocket pods. The engine skill is designed to avoid every type of secondary weapon, including proton torpedoes and concussion missiles.

I think the OP is using the word evasion in place of what most people consider "breaking a lock" (barrel roll, etc).

 

To answer the OP, flight time is a factor that the dev's wanted you to consider. That's why there is an upgrade for the slow traveling torpedoes to increase their flight speed 100%. Sounds like you'll be taking that option at tier 4.