Going to see a big increase in loudness.
The Selenium 6w4ps are some of my favorite 6.5" mids out there. They are cheap, and sound great. Just be sure that you deaden and seal your door if you mount them in your door panel. Makes a big difference. Without it, just playing freeair there is 0 midbass or bottom end. Pretty much no output below 200hz. As soon as its in an enclosure (such as a sealed and deadened door), you can get usable output down to 60hz without much problem. They also play up very high without any fuss. For a while I ran them wide open without a LPF and just boosted the frequencies at 8khz and up a little. Sounded good for not having a tweeter. If you added a nice set of soft domes with them (Id add a tweeter per mid. so if you run 4 mids, go with 4 tweets), but stay away from bullet tweets with the Selenium 6s, unless you have a ton of them. The 6s are loud, but not as loud as most pro audio speakers. They have a more smooth, warm sound, and a bullet tweeter doesnt really blend with them on regular music.