I think the standby for Pirates is much longer now since FP+ than before. But we don't go at peak times so maybe that is why.
We just returned from our first FP+ trip. We had some of the lowest crowds seen in years, so I am not sure my perspective is entirely accurate, or would compare to Christmas time. One thing I noticed is that the FP+ line is catered to over SB nearly 100% of the time. If the CM sees people walking toward up the FP line, they will hold the SB line to let them pass. Then a few SB go, until someone else is coming in the FP line. That being said, we waited in zero SB lines outside the first 1.5 hours of park operation. Being able to modify originally made FP (minus 7DMT, PP and FEA), to earlier times once one FP was scanned, and then the ability to add 4th+ FP (one MK day I think we had 13 FP+) made it so we literally never waited in a SB line. Since people are canceling FP+ every second, the selection changes every second- and I was able to get the time I wanted (which was for the minute I was done with the current ride) for the entire day by modifying. I imagine in peak crowds the availability isn't there. So I still have mixed feelings about FP+, even though it worked really, really well for us. If you do have to stand SB- be prepared to wait a lot longer than you used to.