Any constructive criticism is welcome.
I was going for something that would catch your eye but not too fancy.

That looks really nice, extremely eye catching, good work. It does look too big for some reason and I had to check it myself to make sure but it is as you say 400x200 so is the correct size.
I love the metallic green finish on the KAT logo, the whole thing looks very professional and I think you may well find yourself being approached by people who would like you to make them sigs as well when people see it.

 
We have a lot of very talented sig makers here and once a month there is a competion to see who can make the best sig for one of our Mods so if you have not already had a go you might want to give it a try and test your skills against the best we have here. (It is just a bit of fun but competition brings out the best from people so everyone gets a little better while competing) You can find the monthly competition here if you are interested. (Although this month is now just at the voting for the winner stage now)

it caught my eye,but it's not too fancy

Oh wow thanks for the info :) I'll be sure to enter next time1