It's about 20mm difference between the 240 and 850.
What's the offset & size of the wheels that you'll be using, and what is the tire size? You may be fine with 20mm, or you may want 25mm to space out the wheels a little, and with wide tires you might need as much as 30mm.
No I won't. I haven't bought wheels yet, but I'm thinking of getting something in time for Carlisle. I have a relatively affordable alternative in mind, unless something else comes cheaply before then.