'If your using a seat harness'
Some comp ski / wakeboard floatation jackets are perfect impact jackets. They have been designed for Body slaming & floatation for years.. and at a faster speed than kites will ever reach (drop height speed included).
If you shop round there cheaper too.
My personal crash experience from speed skiing was clocked at 70mph on an outside shoulder turn - bounced in air (length) about 40 feet then end over end slams.. the jacket worked great but got slight concusion nearly passed out

my comfort was in knowing i was not going to sink (flotation).
I still use the same one its great for kite slams !!!
If your not pushing the limits.... a blow up ring will work fine

Cheersz .............. go Hard or go Soft