That is the idea.
I don’t know the exact threshold for switching but I would imagine it’s between -100 and -110 RSRP. The device would most likely make the decision as to when to switch between UL CA and HPUE because it needs to regulate itself to stay within FCC maximum power levels.
The change in L2500 performance won’t be a big, considering the new XS supports HPUE—in fact L2500 performance may even be better on the new phones.
That being said, looking at the EIRP values of 800 MHz and 1900 MHz performance on the XS/XS Max, it looks like L800 and L1900 performance are going to take a big hit. The XS performs significantly worse on those bands than the last generation of iPhones.