It appears that upgrading bullet shooting speed is useless after a certain level. I am playing with a friend and we have the bullet speed up so the stats say the turret should be shooting 360/s. But if I watch a turret in action, it will consume one (uranium) clip per second. The clip has 60 bullets each, so it does 60 shots per second.

This seems to be confirmed by this thread: https://www.reddit.com/r/factorio/comments/4oo4lp/psa_dont_bother_with_shootinglaser_speed_upgrades/ which says turrets can only shoot one shot every tick. I believe a tick is 1/60 of a second so that seems to confirm my findings.

Looks I spent a lot of money on useless upgrades ;).
In the beginning the speed upgrades make a lot of difference, but it is a pity it is capped.
So the only thing to keep up with late game waves is to build more turrets (quickly limited by space) or with the bullet damage upgrades.

Maybe the upgrades in the upgrade window should be capped so people do not spend useless money.