Whelp...mine takes a 27.2 seatpost. (I'm assuming that's what you needed. I don't know the external diameter)
Mine is an XLT and it rides like a dream. The Jamis is a little heavy but it is tough. Their XC frames can handle All Mountain. Their All Mountain frames can handle a little DH/FR. They're pretty dang tough.
The suspension itself works very well. Mine feels like at least 5". (some 5" frames don't feel like 5")
You have a good project there. Enjoy it.