For what it’s worth; you’re spinning almost a 4 inch stroke there, with that thought in mind the more often you get up into the 6K rev areas the more stresses are placed on that engine. Sure, with some work you can rev a Sportster up around the 7,000 mark, but it does take it’s toll.
The higher RPM ranges creates lotsa friction and pressures as well as the stresses mentioned above, these all contribute to the early demise of an engine. The problem as I see it is that once modifications are made it is human nature to go there …often. I have to admit in the younger years, I went there too. But it was much cheaper back then to fix it.