I found my Motortrike rear tires were rubbing on the inside of the tire. I added a 1/4" wheel spacer to eliminate the problem.
I now run P205/65R15 same size as the Triglide. I did add a 1/2" spacer to set the wheels outward for the stance.
The reason I went to the different size rear tire was the availability to get the P225 /60R15 is not stocked at all tire shops if I would happen to have need for a tire replacement on the road.
A problem might come from running a low tire pressure & pulling the tire over in a turn also. If there are worn bushings in the Tri Glide swing arm or bad ends in the pan hard rod you could have movement sideways.
I don't know how much slack is built into the IRS systems, but there are definitely more places to wear.
This is why I wouldn't go to a wider wheel than 6". Also the clearance for the 205/65R15 tire is correct under the Harley fender.
If a person wants to widen out the fender (lots of work) then wider tires & wheels are possible. People have put 7" wheels under the stock fender & say they have no problems. I don't think they have had them in some really hard corners above the posted speeds ! LOL ! There could be enough tire flex to get into trouble with that wide of a wheel.