Actually PF and Penal veterancy is quite close up to vet 3
To the last man is great for prolonged fight from heavy to heavy cover contrary to what one might think originally.
Since Penal get great offensive vet bonuses one has to change the way one uses them and move them to cover later in the game.
Im aware of their accuracy vet but I wouldnd mind to trade some of that for more staying power.
They have THE highest received accuracy at vet 3 of any core,semi and elite infantry in the game gaining a measly 15% through vet.
I like to add, if they do trade accuracy vet for received accuracy (like PF vet), to the last man then can be more targeted to increasing their accuracy as they loose modals. Just a thought.