I don't know. I think they make the first 3/4ths (or even 7/8ths) easier, but I feel like I can't shift gears well in the Dragonfly for a kick because the foam is too soft and you don't really feel the ground. I prefer generic track spikes (think nike rival d, saucony vendetta) for the 800 (and I'm pretty fast for a HSer, mid 1:50s) because of this.
I think the Air Zoom Vics would be better, but I'm not sub 1:50 and I specialize in the 3200 anyway. I typically bank on allowing my superior aerobic fitness to carry me through 2800/3000 about 5-10 seconds ahead, but at NBIN I'll need a kick