By aim time, do you mean time for the towers to swivel around to face the enemy? That certainly shouldn't have to be upgraded; I'll go ahead and increase the speed on that.
If you are seeing too many lines covered, it means that your tower is covering too many path segments. The maximum number of path segments that can be covered is two. So if you have a zig-zag pathway, the game won't let you place towers that cover multiple lines zig-zagging back and forth.
And thanks. :)