Our Muronos are challenged with short suspension travel. If you load it a bit, there is very little upward travel left (hince easy bottoming). If you install a firmer spring you are going to have even a firmer ride. It won't bottom as much, but "most of the time", when there is no load, the ride will be very unappealing.
I have a solution that I have used in a couple of my SUVs in the past. I replaced the rear springs with air bags and a auto-levelling sensor/pump. It enables the suspension to stay in the "sweet spot" regardless of load, and keeps the ride smooth by not having stiffer springs. In fact. I use air accumulators that reduces the air spring compression rate (makes it softer). Depending on how motivated you are, you can put a valve between the air spring and accumulator to change the firmness of the spring on demand. No bottoming under load, and smoother, more balance ride always... Weeeeeee
I have purchased all the hardware to install this in my Murano, but haven't had a chance to get it installed. I live in AZ and it is too hot in the garage to do it right now... Maybe in a couple weeks...