If you are 6'4" and 300 lbs, you will not like this top. Imagine crawling into a NASCAR window every time.
Regarding rain, no roof or windshield combo will keep you completely dry - you will have to go to the full enclosure with doors to do that.
All tops protect you from the sun - just like an umbrella. That's the main reason I got a roof myself. Texas sun was killing me.
Given your height, and I know you don't want to, but consider the Twist Dynamics Gull Wing. It has removable roof panels that open up so that you can step into the SS and then sit down. It's also the tallest one when closed above the seat for the taller people. I am 6' even, 250 lbs, and clear it in plenty of room.