I have Sports Seats in Leather. They are very snug, and hold you quite firmly. It's great getting in, and wiggling until you're securely "held". I'm not big, so feel the seats gripping me all round, really closely.
I have the extending panel at the front of the seat. I find that just a nuisance, and for me, totally redundant. It's especially annoying on the passenger seat, when any bags or rucksacks you grab out of the passenger footwell catch the extending panel, making it spring out and snag your stuff.
I did a 2 x 200 mile journey recently, taking it very easy during the run in period, and thought the seats were great. A big improvement, in my view, on the R56 seats. (Although my previous seats were only part leather - the 10% leather option.)
On balance, I really like them!