can't help you a lot with the 800 single, but three guys in our group are running the monster single on our m1000's and love them to death. One of the guys has the d/d twins on an 800 and swears by them. when you call d/d and you want tech info ask to speak to dale, he has always been helpful.
i have the monster 800 pipe for 700 big bore and love it nice power took a little time to dial in boondocker box and clutch but once done have not really touched setting .