Coming from someone that doesnt ride a super moto but have ridden with them on track on a bike that'll do 130+, I'm not so sure about having a slower rider/bike riding down the center of a straight. I say stay on the race line so you don't have to cut across track to get back onto line to take the next corner coming off a long straight. It's a track day, it on the person passing to make a safe pass. Having someone in the middle would potentially cause confusion and guessing from the people coming up from behind. All the days I've been to, the slower bikes/riders that stay on the race line and are easily passed by quicker riders because they don't have to pick and choose what side they are going to blow by a ninja 300 or supermotard, etc.
There (as I mentioned earlier) are a fair amount of ktm 390's running about these days spread across all groups, they stay on the race line close to the edge of the track giving everyone else more than enough room to pass. Also imagine you sitting there at 100 and two liter's fly by at 180+ (such is the case at road atlanta) one on each side, could startle you, could blow you to one side into the oncoming bike passing you on the other side of you.