Grenade and rock throwing has always been clunky, even with the yellow rendered trajectory line. After some troubleshooting yesterday, I finally figured out why it was problematic: the trajectory line calculation was wrong.
After correcting the code, I also added the ability to detect when the trajectory hits an object, and put some nice sparks there to indicate the point where the grenade/rock would hit the wall. This way, you can easily tell where the grenade will land, and whether it will go nicely through a doorway or even window.
I also increased the base accuracy of throwing, but to get the most out of grenades, you should unlock the first two perks for the Explosive skill.
Additionally, I improved the "low toss" which is when you tap RMB without aiming the grenade with the trajectory. Now, the low toss will go high enough to clear any small obstructions on the ground such as stepping stones, making it easier to toss a grenade into a door.
Finally, when you use the "Quick Throw Grenade" ability which is by default bound to G key, You can hold G to aim with the trajectory.
Now, let's get out and throw some nades :)