I am not in favor or against megs for Sin/SinX, however I couldn't resist on pointing out misinformation provided here.
First of all, comparing SQI of one class to very specific weapon that is exceptional against 1 race and provides near SQI bonus for that race alone is not correct.
so lets compare SinX and Sniper, no other gears but weapon/arrow/endow, both having 99 main stat and job 70. target being orc Lady:
hydra infiltrator fire endow SinX - 1140 average damage (12M zeny)
Hydra Elven bow/arrow fire endow sniper - 1880 average damage (4-5M zeny)
Now compare their SQI weapons with samge stat/job:
4 turtle general twin fang (not very common) fire endow SinX- 1200 damage (200M zeny)
3 turtle general artemis bow with fire arrow Sniper - 2200 damage (260M zeny)
all those numbers do not factor EDP in. EDP with megingjards would certainly disbalance PVP/WOE. and could even disbalance MVP, as even without megs SinX can easily reach 100K SB on most MvP's.
Edit: also regarding SinX and 1K damage, that damage was achieved on demihuman race monster, on other monsters or no infill, achieving 1K damage does require significant equipment investment.