pepole often confuse exhaust velocity with back pressure on four strokes its good to keep exhaust velocity up becyse it will help suck out burnt air petrol and if you have valve overlap it will help suck in fresh air
All back pressure is is a sound wave. Just putting your hand infront of the exaust port causes back pressure. And yes it will have backpressure with that header. Dont worry about it. Theres no evidence or note that says how much backpressure a engine should have.