The only downside to trimming cases much more than .010 shorter than the chamber length is the additional powder fouling and hard baked carbon deposits that will form in the area between the end of the case and the end of the neck length area of the chamber.
Depending on the case capacity, type of powder used, how often you clean, etc., it may or may not become a big deal. I like to run all my cases .020 shorter than the chamber length. But I do pay a bit more attention to this area by using some JB on a patch wrapped around a worn out brush and twisting it in that area occasionally. Works great. Iosso on a nylon brush with no patch also works fine.
One other thing: don't assume that your chamber is the exact length it's supposed to be. You can check the chamber length by seating a flat based bullet backwards and closing the bolt. The base of the bullet will jam against the end of the chamber giving you the chamber length. If you have a Stoney Point tool, you can do the same thing with this.
It's not unusual to find .020 more chamber length that your chamber is supposed to have in factory chambers.
