Would have been able to function after he took the c-virus? It does say that a J'avo will tend to follow his last given orders, and it seems for Piers that was fighting for the BSAA. Do you think he might have been able to live a productive life as a Cole McGrath wannabe?
I don't know. It seems he got the same type of "master strain" that Simmons and Carla both got, but somehow managed to keep from mutating completely like they did.
I would think that he might have been able to stay aware (more so than J'avo or zombies) but it's questionable how sane he would have remained. Mutated Simmons and Carla seemed to completely lose it even if they could still think and speak. Granted, they probably weren't the most stable people, but...
Piers was having a really hard time maintaining himself there at the end. It was only a matter of time before he finished mutating into a complete monster, and he chose to go out when he was still mostly human.
When has anyone been infected by a virus in these games and been better off for it, and NOT turned into a mindless monster? Wesker is one case I can think of, and that was a virus custom engineered just for him. Sherry profited too, but it took a combination of a G-Virus embryo from someone of the same genetic line AND a G-Virus vaccine to get her to where she is.
Even the people who make and use these viruses don't want to use them on themselves, unless they're going to die anyway (William Birkin, Carla) or get sneak attacked (Simmons). They always go on and on about the "incredible power" the virus grants, but then somehow always manage to die to a few humans wielding standard firearms. That's not much of a selling point. Sacrifice your humanity and turn into a monster, then still get brought down by a 9mm handgun? No thanks, BRO.