Maybe you should watch the "engine masters" episode where they did just that and dyno tested each time after progressive smashing. You might be supprised, its interesting to say the least!
Yeah, it's really about total effective flow diameter rather than the shape. You can bash them pretty good and not impact the actual flow cross-section, just change the shape of it.
Kinda like NASCAR that uses rectangular exhaust tubes vs. round ones.
Unless you really change the total effective flow cross-section, performance isn't really affected.