Thanks for the write up, I'm a medium Yelli rider too.
Do you not notice the slightly longer chainstays of the Fireline? That's my major concern in moving away from the Yelli.
Overall- I find the fireline to wheelie/manual easier than the Yelli, but not as easy as the Honzo.
Don't pay attention to the whingers that get all bent out of shape on chainstay length. Here is the truth:
The fireline has a "normal" (i.e. slacker) effective seat tube angle compared to the yelli/Honzo. If the fireline chainstays are long, why is the seat tube so close to the rear tire? It's pretty obvious these chainstays are as short as you can get without making a compromise somewhere else.
In terms of what makes a bike easy to manual/wheelie/slash corners, chainstay length is not the only factor. The fireline has a relatively short wheelbase, slacker seat tube and a tall head tube, things which help make it easy to get the front end up or snap out of corners.
Having owned all three, the FIreline feels in between the Honzo and the Yelli, which is what I was hoping for. The Honzo was super playful, quick and easy to manual but lacked stability on fast rocky trails. I liked the Yelli better, but the fireline is little easier to manual for me with the shorter reach and slacker seat angle. Ride quality and weight wise the Fireline is the best of the three. If you like the medium Yelli the 18" Fireline will feel very similar. Lighter, more compliant, a bit shorter reach when out of the saddle, and a bit less standover.
On my Honzo, I had moved the sliding drop-outs back to 16.75"/425mm to add some stability on fast rocky trails. Helped, but still the stiff back worked against the bike in these conditions. IMO 425-430mm is the sweet spot- look at the chromag and ROS9- both in this range along wiht the fireline.
Perfect? Not quite for me- only because I would prefer a 17" size option. And 142x12 drop-outs. But I am very happy with this frame.
as for the seat tube, I've fit both a 31.6 syncros and a 30.9 Kindshock i950, with a problem solvers shim. Both fit perfectly with no slop regardless of insertion depth.