Crikey. I don't understand what you mean by downhill kit, that's a hugely long bar/stem combination. The riser bar could potentially be helpful in diminishing the effect of that vast stem, but only if you use the sweep to reduce the distance between the steerer axis and the line between your hands. It's really exaggeratedly long at present.
this is retrobike i believe and pre 1997 bikes?
if you remeber rightly, these were how things were back in the day, the bars and stem were in MBUK as being the 'best'. thats when i brought them in about 95/96. rocky mountains were also prized as being strong by downhillers.
i also have a long back and its actually quite comfortable. also the head angle is slacker than modern setups which need shorter stems.
in the 1990's with five others, i did a 25 k downhill route in New Zealand on the same bike with rigid forks and the same bars and stem and it was fine. apart from the arm pump, slickrock, shale and sides of mountains!