the flat spot could be the large DISA valve being slow to open, especially since tunning didn't work.
Try unplugging all the DISA conections, leaving them all open and see how it behaves. Perhaps that could rule out Disa and make you think Valvetronic not opening or throttle body issue, also not opening wide.
Valvetronic & other sluggish issues has been discussed in e90post n52 section recently
128i Sport 6MT converted to Euro 130i spec, tuned by evolve ~220 whp 207 wtq(ft-lb) SAE
In-progress: n54 ssk, m3 trans mnt, e36M lip Wishlist: m3 susp, n55 eng mnt, headers, LSD, finn dif e60
"The 1-series is the last car that BMW engineered before the Germans, as a car-making culture, fell out of love with driving." - R&T 2013 135is