I've had similar problems with broadhead grouping, but not as severe as your's. I would suggest one of two options.

Option 1: Get some high-quality mechanicals that are designed to perform at 300+ fps. With mechs that use O-rings, they often recommend to put 2 rings or rubber bands on for fast setups.

Option 2: Get some low-profile broadheads that should work better. Slick Tricks are the best I've found at shooting right with my field tips, but there are some other heads out there that probably work just as well. NAP makes a couple of low-profile heads that are supposed to shoot well out of fast bows (one is called "Triton", I think, I forget what the other one is called).