just put on the 3" extensions on my 08 pitbull. at 6'3" i felt like it made a huge difference.........in comfort that is. didn't notice much in handling.
is was able to stretch out the brake line so everything but the extensions remained stock. the extensions had a kit that including an extension for the shift linkage too.
at 5'9" the 3 extension should be more than enough for you. personally i think the 6" extension is overkill. like you said at high speeds you might feel like your ready to slide off the back of the bike. that and you'd be reaching for the foot controls when you take off.
GET BUSY LIVIN OR GET BUSY DYIN.