To fix the issue of flat spots at 2-3000 rpms under acceleration, I got a hardly used fuel rail off a 2002 MX-5 1.8 and swapped my red 260ccm NB Denso injectors (which worked fine until the 4-2-1 header install) for the lilac 265ccm NBFL version.
Supposedly only run for 62k kms / ~40k mls, which i can believe since the fuel rail looked like new.
The "old" and the "new" face to face
And in they went...the installation takes maybe 15-20 mins without any rush.
Since I had cleaned and tested them before, the car started right up without any issues. A quick test drive revealed a better acceleration in the beforementioned rev range. I didnt have the opportunity do drive for a longer time yet but it's definitely an improvement.