I see Big Green has introduced the 17 Remington Fireball for their CDL limited edition. Is this just a necked down 221 Fireball?

its on a fireball case, but it's unknown yet just how close it will be to the 17 mach 1V, and whether or not you can use mach 4 dies etc. has to get along a bit further yet

Check out the thread in Varmint Hunting; side by side pic of a 17 MIV and the 17 Fireball.
The 17 FB data indicates a slightly longer neck (0.020"), and it may be a bit larger in case neck o.d.
Hard data should be out this week (SHOT Show starts Thurs in Orlando).

I'm waiting on the Model Seven CDL to become available in this caliber. According to "Big Green's" website the .17 Fireball will be available in this model. The barrel is 4" shorter than the CDL SF Limited. I hope this doesn't effect performance too much for it makes for a much handier rifle in my opinion.