You mentioned interior portraiture, I'm assuming you mean environmental portraits which is what I do. I have owned a P40+ and currently my work horse is the IQ260. I own a LS55 and it has been used with both cameras and the results have been excellent. I tend to shoot at the more shallower f-stops say 2.8 to f 8 and focus on the subject eyes for my style of portraits.
If you also shoot portraits outside, a leaf shutter lens combined with a high flash sync gives much more flexibility. When the back ground is not desirable or distracting, I simply select a high shutter speed such as 1/1000 and set my fstop to 5.6 (of course with radio remote such as the Profoto Air sync) scrim off the subject with a silk or black flag and pop in a bit of strobe. Of course I'm choosing the location, so it won't be at high noon on an Arizona highway....
My photography website is full of portraiture, even action portraits taken with the 55LS. Not sure what your style is, but its been great lens to have in my kit.