I replaced every speaker, 3.5s and 6x9s and sub. See details in signature.
I had about a 10% decrease in overall volume from the 3.5s. About a 25% increase in volume and bass response from the 6x9s.
Overall I am much happier with the sound quality of the infinity vs stock HK speakers. The mid and high range is smoother, clearer, and more detailed. The highs in particular are softer and less shrill which I prefer (that's a personal thing and if you don't know what you like, I'd encourage going to a car stereo shop and having them play some samples of songs YOU like to get a feel for how they sound... You'll know what sounds good to your ears. Don't get to caught up in volume alone. I like music to calm me, not make me irritated, so I lean towards silk or textile tweeters).
I had to lower the bass on the factory EQ. The 6x9s were much more efficient and responsive than the factory ones. I'm at -6 or -7. It's too much mid bass otherwise for my tastes. I fine tune the sub with the PAC AMPPro remote volume knob and am very happy overall. Might add a 2nd 10" to fill in that low end, but I can definitely live with where it's at now.