So I've been tinkering around with the new genetic/cybernetic options. Right of the bat I went with the genetic's lab because I like the idea of running a squad full of super-soldiers more than a squad full of battle-mechs. The gene enhancements can be added whenever because the unit keeps their class and levels as normal. But the description for the MEC says they lose their class when making the switch.
So, do you guys think a Fire Emblem like strategy of fully leveling before upgrading would be ideal or will MECing soldier immediately generate one as powerful as if I had waited?
The MEC Troopers are not an addition (like Psionic), but instead a new class. Augmented soldiers retain their rank, and are given two skills to choose from each rank (just like the regular soldiers) and gains a special skill based on their previous class. I don't think waiting will make any difference, unless stat growth is tied to using a MEC (though, since there is no special piloting skill, I'm sure each class will give it's own benefits).
You do have to remember though that the MEC Trooper also needs a MEC suit to function in combat. They can equip different MECs and are not locked in to one. Each MEC has to be built and upgraded individually. This means that a maxed out MEC trooper may have great abilities, but no weapons early on.
When a soldier is converted to a Mech Trooper he retains his Rank, Will, and -- most relevant to the question -- his Aim. Mech Troopers have terrible Aim growth.
Total AIM growth from Corporal to Colonel: Heavy = 8 Mech Troop = 10 Assault = 19 Support = 20 Sniper = 30
To have a Mech with the highest potential AIM, level a Sniper to Colonel, then augment him.
Mech Trooper actually has better progression than a Heavy (same through Lt.), so if for some reason you prefer the Heavy augment bonus and "must" augment a Heavy, you can convert a Lieutenant or lower... all other classes it's best to convert a Colonel.
This is speaking strictly stat-wise... things can be situational.