I am not an expert on the 6BR improved rounds by any means, but I doubt you will lose much if any accuracy with them. Most will cost you a bit of neck length, which could limit you a bit in bullet selection. If you go with a long throat for heavy bullets, you may not be able to run the shorter bullets and still get into the lands, and if you go for a short throat the heavy bullets may end up inside the case reducing the capacity. But you will gain extra case capacity and velocity, which makes it easier to reach 1000yds accurately.
You will have to fireform all your brass, which adds extra time and work. And that can be a pain if you are short on time.
Truthfully, unless you are going to be shooting past 600yds most of the time, the 6BR will probably serve you just as well without any of the hassle of the improved rounds. You can use brass as bought with minimal prep, and keep the long neck for improved versatility. It is very easy to find an accurate load for, and needs far less tuning than the 6PPC usually does. Basically, if you can't find a good load with H4895, Varget, RL15, or a few of the other useful powders, you probably have an issue with the gun. My buddy spent 1 day testing loads, and was getting exceptional accuracy pretty much right out of the gate. He didn't even bother trying to improve it after that, as it has been a consistent .35 MOA gun ever since, and it's a bone stock rebarreled Savage. So it's not hard at all to achieve amazing accuracy.
If you want less work reloading, no fireforming, and plan to shoot inside 600 yards most of the time, the 6BR will do all you need and then some. If you want a 1000yd gun, than maybe look to a Dasher or BRX. The stock 6BR will still make it to 1000, but it will have a bit more drop and drift due to having less velocity.