Ive had the SB23NRXS45-4 mid woofers in my car for about 3 months now. My experience is that they live up very well to the specs and response graph provided by SB Acousitcs. They handle the area between 80hz and 1khz VERY well in my normal door install. They could probably go lower in an actual enclosure but in my install they fall off so rapidly in the sub bass region that I cannot use them at all musically for anything bellow 60hz. This is actually a good thing because I had many issues with other midbass drivers being limited by bass frequencies. And even with a 24db high pass these other woofers were often limited by sub bass frequencies like 55hz. With the steep nautual rolloff of the SB23NRXS45-4 and its high xmax and surface area; I have 0 limitations due to low frequency mechanical noise. And this is at very high volume. I also experience very low distortion durring high volume mid bass heavy passages. This again leads me to believe that the SB23NRXS45-4 lives up to its 13.5 mm peak to peak xmax rating. As for the upper ranges well as I said they work VERY well up to 1khz. In my car I have them running up to 2khz in a 2 way. And I have noticed the effects of both the low crossover and the off phase dropoff. According to the graph by SB Acoustics they lose about 5db at 60° by 2khz. And about half that at 30° off axis. This sounds about right for what I am hearing. My tweeters are excelent and having them down to 2khz is a bit of a treat in itself so no issue there (obsidian audio ST-1 if you know them). One reason I wanted to try these 8 inch woofers is because for whatever reason this car always had a really harshly loud response in the 2k region. Which was fine and made some vocals and piano really shine but for most of the music that I listen to the strong steady snare drum would really hurt at high volumes. I have never had this issue before with any system so Im pretty sure it has something to do with my cars doors. Either way the SB23NRXS45-4 and its off phase rolloff at 2khz did work to tame this issue a bit for me and made extended listening at max volume alot more of a pleasure. I am running the tweeters with a little less gain so they cross over at a similar volume to where the 8 inch woofers end off. All in all I really love this setup. I picked it out based upon what I personally wanted: ultra high midbass capabilities, and to tame the 2khz region a bit.
As I said it has its tradeoffs: a low crossover point, and some off axis dropoff in the 1.5 - 2 khz region. But it is an absolute pleasure to have midbass that can produce low frequencies well enough that I can keep my subs on the deep bass where they shine.
I havent had any thermal issues with them. I live in south florida and it rains alot but my car is fairly new and I don't think that much moisture has really gotten to these woofers. They do feel like they have a paper cone though so that might be an issue in wet conditions. Can't really speak to their miosture resistance untill I have run them for a year or so but so far so good.