There are alot of variables here which mean you cant get a real answer.
In general:
If you get lower temps without a side panel, you dont have proper case airflow.
Changing the cooler can have massive changes on temps, depending on the current cooler and the new one.
Changing thermal paste can have big effects, or none. Depends on the current paste, cooler, how old it is, how it was applied, etc. (Assuming the current paste is installed properly new paste could have up to 3-5C difference, at most)
its a cheap case from chinaWhat case do you have, and whats your fan setup like?
Cooler blowing air to an exhaust is good, more intake could help depending on your setup.
no way to know for sure. every system is different. obviously a better cooler will give better temps. replacing the paste helps lower temps as well depending on the age of it. a couple years won't dry it out as much. but 5 year old paste will make a good difference once replaced.
check ut a review or 2 for the new cooler you want and see how it performed over a stock cooler. i'd assume you'll see similar degree drops but not exactly the same. don't look at specific temp as you won't see that, but look at how well it did vs the stock cooler to know how much it might lower temps.
my case has space for 3 front fans, u think that would solve the issue?What case do you have, and whats your fan setup like?
Cooler blowing air to an exhaust is good, more intake could help depending on your setup.