As a big advocate of Ultra (to the point where my HVZ power games involve an Ultra Amp), I think you'll struggle to find many folk willing to mod Ultra to continue to fir Ultra darts.
Most ultra mods swap out to short darts, particularly the flywheelers.
My best recommendation is the 15 dart extended mags you can get from China (Ali express IIRC), they need a spring adjusting and the follower lubricating, but they work fine afterwards!
Edit: spelling
I only have talons and tachis, so i have not tested Kodas, i believe it's supposed to take them though?
I haven't, I only have straight talon 15's and tachis, both work fine tho.
I only have Worker gen 2's, but the dart zone pro darts average about 140 with the SCAR on (127 to 146 over 8 rounds) but with the SCAR removed it hits 160 pretty consistent.
I'd recommend swapping to a talon or Koda mag tho, the stock one is a bit tricksy
I get 160FPS out of mine out of the box with the short darts it came with.
And my axe