I've been to the DPM storefront years ago when I visited Athens, Greece. It looked like a small machine shop. The people there were very nice, and I purchased a DPM system for my PPQ. I didn't stay too long because it was already dark out, and the storefront was in the middle of an alley.
I never put enough rounds through the DPM system to get comfortable with carrying the pistol with it installed, but with the few hundred rounds I put with it installed, it didn't malfunction. I'm not a big fan of recoil reducing guide rods. I've seen and heard of more than enough reports of them failing to function that I choose to stay away for the most part. The DPM system for the PPQ was purchased more out of a curiosity than anything else.
If you do choose to install one in your pistol, realize that every report of proper function and the entire track record of these pistols may as well be thrown out the window since you are replacing a part as important to function as the recoil spring, and you are replacing it with a more complicated system that is not used in the vast majority of PPQ pistols out there, and doesn't have the same track record. If you hear of five issues with DPM systems and your hear of five issues with the factory system, then there are many more issues with the DPM system as a whole since there are so many fewer in use. Do your research.
I'd advise against using one spring for practice loads and another different spring for carry loads. One of the purposes of practicing with a carry pistol is to get used to how it shoots, and to prove its reliability. Switching between two springs (or three that came with my DPM system) would be the worst combination in my opinion. You are basically practicing with a pistol that will have a different recoil impulse than the one you will be carrying with the lighter spring, and with a pistol that will have that many fewer rounds through it in the configuration that you carry it in to prove its reliability with the heavier spring.
I think most people would be better served just sticking with the stock configuration. A lighter recoil spring is preferable for reliability with a weak grip, an awkward grip, or an injury. Reading thread after thread here on this forum about how people try to save money by shooting steel cased ammunition leads me to believe that most people out there aren't going to be buying defensive ammunition by the case to prove how reliable their pistols are with the heavier spring.