Won't flow any better without the catback, the exhaust doesnt mean anything past the downpipe. And being that its a 3in DP, there is no restriction.

I would stay out of boost if you are seeing KR, plus you don't have any idea whats done in this tune and it might not be safe with a 3.5.

So for right now just drive it normal, but if it was me I wouldn't even drive it at all because open headers will just make my ears hurt.