Hi Punker,
I fitted hagon Nitros on my 2014 Roadster a couple of years ago. As you're in the UK give them a ring and they'll happily talk things through with you. Dave is the suspension guru and has worked there for many years. Knows the Rocket well.
They build them to ride as you do either mainly solo, 2 up etc and take your weight into consideration and set them up ready to fit. IIRC they have a lifetime guarantee. Mine came within a week of discussing them.