Well, a few things I would like to add

1.) Don't shoot thru brush. Unpredictable things can happen. Quit doing that
2.) 150gr should be faster, not slower with all things being equal
3.) Bullet construction is more important speed

I use 165's in my 06 and 308. Because they are the most accurate bullet I reload for. I also use Sierra Gamekings and Bullistic tips which expand easier.

If your hitting them thru the lungs and doing alot of tracking, check out the wound. If its a nice exit and lots of trauma to the vital organs, then its not your bullets or speed or shot placment. A 243 sure won't down a deer faster than an 06. A bullet doesn't do anything magical. It causes trauma and shot to the vital organs, and after that blood loss.