If the turbo isn't out of steam and you're keeping the power down because of the stock block. Why are you even concerned about a 3.5in exhaust for more power? You have more power on tap, you're just not using it.
Get some denatured alcohol from a hardware store and throw it in. If the knock goes away or reduces, it's real knock. It could be noise, in which case the knock would still be there.
From what I've been reading, the 2.5 bottom swap is cheap and relatively easy. But, You are sacrificing the strength of the MZR DISI for a cheap bottom end. It'll get you back on the road, but I would use that purely as a crutch while the 2.3 gets rebuilt.
To piggyback off of this, when you correct your gravity, correct your scalar as well. The scalar alone is adding about 2% more fuel that needed if the gravity was correct.
Im running a GT2871, I love the little thing. It's enough to max the stock fuel system and enough for running around on the streets.
Head lights look good! gotta a clear coat or something on there to keep them clear.
Yeah, sharp tones. Not much louder I don't feel, but I like the tone better.
I've been real happy with my STR.t's. I've got mine paired with Cobb springs.