Hi Adrian, I fitted the 860 about 500 miles ago. No tuning required. I rebalanced and set the timing, but the jetting feels spot on. Mine has quite large main jets, needles at top position as stock and identical to R80 settings, so no surprise.
Regarding dual brakes, luckily mine came with them. As standard, they are adequate/good, not fierce by any means. The 860 has transformed my bike and I too would want to upgrade fairly immediately with only one!
The other option is a different caliper or bigger disc. This seems to have been done a few times. R65 discs are quite small. I've done this on several cars as I tend to use brakes on the country roads I take to work. A longer lever / diameter is noticeable in increase in power. It will be quite obvious on the small front wheel of the R65 though.