Having had all of the above and then some on my own car. ......
Bilsteins worked well but definately hit a wall once you move much over 300-325 springs.
Koni single adjustable, IMHO, ok for stock springs. On coil overs, best with conservative spring rates only or ride n handling suffers hard.
Koni Dbl Adj, good grief do these things work well with aggressive spring rates. My car with them + 450lb front springs outride Bilsteins with anything over 300lbs. On the track, nothing but praise for them. Well worth the extra money.
If I keep the Cobra, next step will be JRI on all four corners.