No.
If there is evidence to support this theory, I guess I must have missed it completely, and considering that I pride myself in catching the 'little things' in BGC, that's not too likely.
For those who don't know, there has been a rumor on the 'Net for years about Priss being a Boomer. It makes it into pages every once in awhile but almost no one believes it, save for a few poor idiots who have no grip on reality. The rumor is based for the most part on Priss' physical endurance level which, while astounding, is not really any greater than Sylia's or Linna's. More attention is given to Priss' agony than is to Sylia getting her back damn near snapped in half, almost losing a limb, practically getting her neck broken, just about getting her head cropped off, et cetera. Priss does not go through a whole lot more than the others (Nene aside) in the physical pain category.
The 'physical endurance' thing is the only tiny, miniscule bit of evidence supporting the idea that Priss is a Boomer and that can be easily dismissed by the above and the fact that BGC and its creators loved to glorify Priss to a truly disturbing point by having her do things that no one could live through and then making everyone cheer and clap for the heroine. Not to say that Priss doesn't suffer, but it was unrealistic and was obviously meant to be so.
Probably the most important idea settling this argument in the favor of those who say 'no' relates to the above. BGC loved to make Priss unhappy. Wouldn't Largo have made some mention of Priss being a Boomer, or someone else have hinted at it, at some point? Nobody did.
To be honest, I am not opposed to the idea of a Knight Saber being a Boomer; in fact, I believe that Sylia Stingray is a Boomer. But there was so much evidence to support that while there is none to support Priss being anything but human. While Sylia was the extreme of what is decidedly not human, Priss was the extreme of what is widely considered human. She was emotional, reckless, illogical, and passionate. True, Sylvie could be passionate, but Sylvie was logical right up to demanding that Priss kill her. Even the Sexaroids, built for a sense of feeling and emotion, could use plain and simple, stark realism, whereas Priss failed to be realistic about anything throughout the entire series.
Another interesting factor is that Largo referred to Priss as a vulgar human, a worm, a dog, and so on. Largo was the 'Boomer Messiah.' He misused Boomers but he never outright insulted them because he was one. Had he stopped Priss and said that they were two of a kind, I might seriously consider Priss being something other than human, but he never so much as brought that forth as a thought and thus the one most brutally honest character in the series has proven what I and most other people have thought all along: That Priss is not a Boomer.