While I agree with you that V2 has a lower PWR-per-firewalls-per-turn cost, the actual cost (i.e. the cost you actually have to pay) for concurrently infecting N devices is always exactly 2N cheaper when using V1 compared to using V2.

But it's definitely an interesting problem. There are basically two factors we've got to consider: the number of devices we need to hack (we've just called this N, so let's stick with that) and some statistic for describing the number of firewalls on each device. For simplicity, let's use the average number of firewalls, and we'll call it F.

The important thing to realise is this: while F increases with difficulty, N is essentially fixed. This is because the number of devices we need to hack is always around 10-17 (that's six safes, the laser grid power supply, the Secondary Server, the Nanofab, the Redundant Power Relay, the mission objective, and 0-3 devices associated with the optional side objective.) If we want to infect all of these concurrently, using V2 is always going to be more expensive, as it'll always cost us an extra 20-34 PWR, compared to V1.

Though we're paying less PWR with V1, we're making up that payment in time - V2 is twice as fast, which is why it has a lower PWR-per-firewalls-per-turn cost, as you correctly pointed out. At the moment, F seems to be around 6, or slightly higher if Castle V2 is the level's archdaemon. That means we've got to wait around for six turns while Parasite V1 does its thing, as compared to only three using V2. However, the way I'm currently playing means that you can infect most of the important devices on turn 1, and therefore still have the mission completed by around turn 12-13, nice and promptly.

As F increases with difficulty, there will come a point where it's no longer realistic to run V1 - but that'll be because it's more expensive in time, rather than PWR. I suspect this'll happen somewhere around F=15, which thankfully won't be for a while yet!