Swing speed should not matter. It's more about head design. If you have a severely steep swing path, the larger sole of the T400 will create greater bounce and help slow that fat shot from being so severe. They also have more offset so someone who has trouble getting their hands ahead before contact might benefit from the T400. If you want the combination of forgiveness and some maneuverability, the T300 will be better for you than the T400. If you think the T300 is not as forgiving for what you need, seek out a local shop that carries Titleist and maybe they have a T400 demo iron you can try.

Thank you for the help. I am trying to find a shop that has a backwards T-400 7. I am left handed.

The T400 launches a little higher. Hard to compare 7 iron to 7 iron with the difference in loft. With a larger head and wider sole, I had difficulty hitting from deep juicy rough compared to T300.

The P is/was 39 degrees, so I also carried the 43. I prefer the feel using a 48 Vokey. If you live in a northern climate the revised v2 T400 should be coming out soon.