My experience:
Voided warranty = no
Increased mileage = not very noticeable either way and I really wasn't concerned since I run B-100 for the last 400k or so
I did EFI Live because it enabled me to tune in a turbo brake on my 2006 and allowed us to avoid the dreaded sooted/clogged turbo that plagued the 2007.5 Cummins. Gonna be 9 years and they're still running strong and have good oil analysis from Blackstone.
Edit: I can't really claim B-100 for the whole 400k because we have to blend during the cold weather.