Big tube headers will definitely not help spool up. Think about it; a turbo exhaust manifold needs to be pressurized to spin the turbine and make boost. Adding volume (big tube headers) means it takes longer to build that pressure. Don't buy into reversion or scavenging theories with these systems. That is naturally aspirated technology applied to a turbo application and they are VERY different in design to a turbo manifold. There are books on this and I highly recommend anyone interested in learning about the difference to go hit their local library or Barnes & Noble.
Changing to a smaller A/R housing on that turbo will not net you fantastic results. The 3076 is better matched with the mid or large turbine housing, not the 0.63. You'd likely get a bit less power but MUCH better spool up characteristics and low end by going to the GTX3071 with the 0.82 A/R housing. Then again, if you're using an ATP style OEM housing you're kinda stuck with few choices, unlike using the Tial housings.
The ATP housings are very convenient as they are a bolt in product, but the housings are still based on the OEM housing design, which without a doubt reduces turbine performance. An EWG version definitely induces turbulence into the volute, which is not good. Moving the WG further from the turbine and using a performance designed housing (Tial for example) will net you some decent improvements.
Changing to the smaller MV-S (38mm) wastegate will give better modulation of boost and less of the uncontrollable surge of torque that is causing you to loose grip. The smaller gate can allow it to crack a bit sooner assisting some with spool as well. You're not running low boost pressures so the 44mm is not needed. In fact, we NEVER use a 44mm on these cars, it really doesn't make any sense unless you're running 12psi of boost.
Another option is you run some fuel with more bang to it. C16 and especially E85 would widen that curve quite a bit AND make more power.
I can't answer if the Killer B Motorsport header is the right header for you. Everyone has their own reasons for what their wants, needs, and goals are. I can tell you it makes more power than anything else currently available. Or at least we've not come across anything else that makes more power (and there are LOTS of plots not floating around out there), and using titanium alloyed stainless steel (321) certainly has it's benefits as well as a hefty cost of admission. Unfortunately, there's nothing we can do about the costs, it is what it is. The justification of the product, or any product for that matter, is what it's worth to you (performance, durability, etc.) by having it. For some it's worth every penny, and for others it's not as important.