Nice work on that setup. How did you secure the front panniers? Is there a rack attached to rim brake bosses? That looks like quite a bit more weight than two water bottles and cages. Makes me think I'm worrying too much about the handling effects.
Ya that is more weight then water bottles there is a rack in there and it is just attached with hose clamps. Adding weight will change the handling but it doesn't take long to get used to most anything.
I'm currently using a hardtail with a big saddle bag, but I tried an OMM rack on a FS once. Man, did it screw up the way the back end tracked! Enough such that I was worried about the wear and tear on the pivots.
That is my current set up Epic bag on the seat and then sleeping pad is strapped to the bars.
I did a road tour once with that same seatpost mounted rack. It worked pretty well with about 10 pounds, but I neglected to counter balance with weigt, so the front end wanted to lift. Not good in the wind. Did you have good luck with yours? Do you have an idea of the weight back there?
I would say I had a least 20 pounds on the back rack at times even more when I filled that big water bladder the rack broke on the trip and then the frame cracked latter. Not a good way to go.