From the specs I found for the Reno, the curved row of 3 is only 4" deeper than a straight row of 3.
I have a different Berkline model, so these numbers might not match the Reno exactly. In a fully reclined position, my seat (Berkline 13175) measures 5'-10" long from top of headrest to bottom edge of footrest. Like you, I'm 6'-5" and my feet hang off approx. 6", so I need 6'-4" min. between the back wall and the seats in front of me. Add to that the extra 4" for a curved row and I would be right around 6'-8". This would be the absolute minimum, with the headrests brushing up against the back wall and the soles of my feet brushing up against the heads of people in the front row. You'll want a little more than that in front and behind just for a little buffer. If you have rear surrounds and want the guests in your back row to enjoy them, you'll need even more. My riser started out at a full 8', but after putting 5" of acoustic treatments and fabric panels on the back wall, I was left with 7'-7" of usable space.
Your screen must be really high if the riser calculator only recommended a 6" riser? Are you sure 8" will be sufficient? 8" is a funny height because I think it just puts you into the category of needing a step, but what you are left with are two very short steps. If you have the headroom, you might want to bump it up to 2x12s, at least around the perimeter. You can always use 2x8 joists to keep the cost down. Double check your sightlines to make sure you have sufficient height.
Regular pink fluffy insulation is fine inside the riser.
No plywood required on the bottom of the riser.
I prefer the step on the outside of the riser. Seems like less of a tripping hazard.