I main sniper so maybe I'm a bit bias here.
1- Lower the HP bonus of tao-gunka card back to the original
No. Snipers are fine as they are in their HP right now. Normally i reach up to less than 300k HP without sacrificing damage. Reducing the HP buff from Tao Gunka will make the class die easily or sacrifice loads of damage.
2- Make the golden thief bug card only partially reduce magic damage on snipers
No. Switching to GTB sacrifices the sniper's reducts and they become very vulnerable to physical attacks. Tanking magic attack and physical attack while wearing gtb with this effects will make this class dig up his/her own grave and if you ask me I won't wear GTB anymore if it has this effects.
3- Make the Thara frog card and usakoring card only do - 15~20% damage on Bow user classes.
No. Distance is a sniper's friend once it is eliminated, sniper's become very vulnerable to anything. Reducing the % damage from usakoring and thara for bow classes only is too specific as well.
4- Reduce damage inflicted when a shield is equipped.
No. Damage inflicted by Sniper's right now are ok but not really overpowered. Also to get great deal of damage you have to be skilled enough to change arrow elements depending on your target. People are still able to tank sniper's damage with the custom changes.
5- Remove the Increase weight ability from all weapons.
I carry some bunch of stuff like traps, speed potions, aloe vera, food, sometimes verters, removing this will lessen the amount of consumables I can carry so I may have to say no. Lessen it to a lower level maybe is acceptable but as the others said weight becomes very crucial on LMS reducing this will make LMS dominated by strength based characters as they can carry more consumables as to the other classes.
In summary, this class was underpowered before and since the server is aiming for class balance they came up with the custom changes and this made sniper a playable class but not to the point of going to the dark side (overpowered). Removing some/all of the custom changes will make this class underpowered again and less people will play this class.