Looks like you're right with the neck through XPs... are there no bolt ons too?
15mm* is ok to me, 14 on the L25 starts to get tricky but since i run .040-.126** strings the difference is not that big!
*Mine shows up 16mm... bizarre!
**.050-.135 on the Spirit and the Warwick
Mine may actually be 16mm - I was measuring with a cheap wooden ruler. I haven't pulled out my micrometer yet.
The original XPs had a bolt on. It's confusing because Gibson didn't assign a new model name to the Spirit version of this body shape like they did with the rest.
Also, I just updated the links in the Spirit megathread - the archived MusicYo pages answer a lot of questions. http://www.talkbass.com/forum/f8/steinberger-spirit-megathread-363051/
