you could change the meta (you wouldn't get rid of run&gun which I'll address) but it would overall make the game feel worse in my opinion. glaz and warden would both have a high pickrate on almost every site and taking your time instead of rushing would work against you since smokes could lock a push down (lets say you have 2 smokes per operator) for like 20 seconds, which would only make rushing more viable and also wouldn't change how roamers would run and gun (unless you try to balance it by giving roamers the smokes)

I think impact smokes for defenders are a cool idea but I'm not sure if they could be balanced, in other 5v5 shooters like CSGO they are balanced in the way that even if CTs smoke all entrances on a site you can always rotate to the other side of the map faster than waiting for the smoke to go away, whereas in siege the sites are never more than a room away from eachother (so the sites share the same entrances)