I don't know about the Calmini's riding stiffer, mine still feels great but I haven't driven an Xterra with the SLR pack for any length of time either.

I left my load leaf on and it doesn't seem to affect the amount of flex, the wheels stuff into the fender enough to rub the fuel filler cover. But I still recomend taking off the load leaf because I've gotten hung up on rocks with it.