I think the best thing to do is simply decrease the aim time and cooldown so the PTRS can be fired faster. It is a semi-automatic weapon after all, and it doesn't hold a candle to the Panzerschreck.
The problem with the Penal PTRS, however, and why it could maybe have 3 is that the Penal PTRS squad doesn't scale really well into late-game. It's not incredibly dangerous to armour, it's snare while potent - is short-range and you're giving up an 8 population 300 manpower tier-locked squad to light anti-vehicle work that falters against infantry the moment it takes losses and the unit often doesn't have enough time to sustain DPS before getting butchered.
I can see the need for increasing rate of fire, but at the end of the day, it's essentially a dedicated AV squad so it does need to threaten vehicles later in the game.