Can't speak to the endorphins, but I ran in peg turbos and next natures and currently run in the plus and regular peg 40. To me, the peg plus feels like a more responsive and exciting next nature rather than the next iteration of the turbo. It doesn't feel as responsive and agile as the turbos did, but it's been a few years so I might be misremembering. The plus is still much softer and pleasant than the next nature. I got two pairs of plus which I rotate and save the normal pegs for recovery runs or rainy muddy days. All in all, I like the plus for tempo work (5:40-6:10 pace) and easy runs and have taken them up to 18 miles.
Curious to hear how the plus compares to the peg 41, as I'm thinking about getting the 41 to replace my 40.