In using a combo power, you aren’t able to gain more combo during it. However, if you upgrade combo power length, once the regular time for an un-upgraded combo power ends, you are still in combo power and are able to gain combo. This can basically break the game for survivor, considering is combo power features invincibility, leading to infinite invincibility.
Also, if not implied, the hud and crosshair dont change timing in conjunction with the combo duration length upgrade.
Yeah, just played a full game of survivor and everytime I lost my ramming shield, I had it either almost back, or already was able to use a new one.