The armor upgrade doesn't seem like just welding more armor on, otherwise there would be a fixed number that the armor would increase by (+10,000), rather than armor multiplying. (1.5x, 2.25x, ect.) Personally I think it's like stripping away steel and putting in titanium. I think fighter armor is fine, because it's lighter armor designed to take up less space, but more pricey because you're using lightweight alloys rather than 300 cargo dense armor. It's a matter of personal choice.