With silencers in not much louder...a little lower but not loud by anymeans...haven't taken them out yet....next weekend project. With them in there was a noticable boost throughout the range. I'd be curious to know the dyno differences with silencers in and out. The header pipes are fat fat fat. The heat shield is replaced with a carbon piece that is smaller than on the stock. Lots of visible welds on the pipes but overall quality is high. Not sure on the o2 sensor question. Seems to have calmed the stalling issue down as well. On start up the scrolling text changed to 1098 Racing Evo from 1098S or whatever it said before. The healthy piping has me wondering if there is much if any weight reduction going on. Only had it above 6K once or twice and that's where the difference was most notable. Doesn't seem to have lost power anywhere based on the butt meter. This bike is deceptively fast which makes me feel like it's slower than my previous tlr but then I look at the speedo and realize it's much much faster in reality.