The difference is quite big, imo. However, none of the listed issues have anything to do with your monitors refresh rate. 144hz makes everything look smoother which can increase performance, especially with heros that require very precise aim (Widow, Soldier, McCree, etc.).
As for your issues:
If youre not even behind the pillar on the killcam thats most likely favor-the-shooter.
Thats usually because of odd map geometry blocking your shatter or because they werent in the cone shaped AoE. Dont forget, even Zaryas bubble blocks Earth Shatter for everyone standing behind her.
144hz is overhyped in my opinion. All it does is help you react slightly faster, and makes the game a lot more smooth and graphically/aesthetically pleasing. I mean, i am never going back to 60hz, but if you play and have played on 60hz your whole life, it isn’t worth the switch if you dont have extra money to spare. It is a luxury, to say the least. I have had my 144hz monitor since christmas and have climbed slightly but i wouldn’t say i improved because of the monitor. That’s just my thoughts on the matter though.
ive NEVER had the luxury to play on a monitor before, and i mean EVER…tbh i cant stand looking at a 19, 22,24,or 28 inch 2 feet in front of me, so since the first day i have ever played on PC, i have always used a TV so i cant experience the joys of this “hz” thing…i dunno what my hz is on my TV but i have no problems playing. my wife use’s a monitor next to me i think its 144, but tbh, looks the same.
i can say i wont need a monitor for my experiences i still play at a decent lvl of gameplay without it so im sure it really wouldnt add much gamewise… if i could go from diamond to GM from a TV to a monitor…well maybe ill consider it but until then all games i play with or without max settings till look amazing and playable without issues to me