850 Ultra XP lean bogs when secondaries open
those ship with extra secondary throttle arms, with various bends, that change the ratio of when the secondary starts to open.
I typically go 1:1, and square jet my "big" motors... but i understand for street use that may be excessive.
Regardless, throw the 1:1 linkage arm on it, and see if its REALLY stumbling when the secondary's start to open )and it gets a bunch of air) , or not. at least you'll have data to compare against if you throw that 1:1 in there and no change, vs it helps / hurts. That should take 5 mins.