I have the Airhawk R and yes, on tight turns it can feel a little loose. But, the tradeoff is I can ride two more hours without my butt killing me so the trade off is worth it to me. I haven't fallen off yet.
If I ever decide to have a custom seat built I will go to Tom Simmons and have him build one of his "trike" seats for me. http://tscustomseats.com/gpage.html
Tom is one of the few true custom seat builders left.