Is this useful for guns that only have a 2 bullet clip? (Which seems to be the majority of them...)
It seems either a damage increase or a charge time reduction would be better in almost all cases, unless it was applied to a high clip size gun like the Antigua and it's branches.
With that assumption, is Shadowsun Setson and Slicker the best 5* in that line?
ASI decreases the time it take to fire successive shots. When you have enough ASI, the reload on guns isn't much of a problem because you spend less time overall firing a full clip. The shoot-pause-shoot method is safe but you have a lot more versatility if you can unload a full clip without stressing over reload.
The Antigua series gains the smallest benefit from ASI because it has a naturally fast firing speed. You would only notice a difference if you were able to empty the entire clip on a regular basis. The Antigua series has such a large clip size that firing a full clip won't happen very often.
There are only a few guns where CTR is ideal. Off the top of my head, you would want it with the Catalyzer and Autogun lines.
The Shadowsun set is a matter of preference. Ignoring the weakness to poison and curse, you have decide if you are willing to play with the shoot-pause-shoot method in favor of more damage.