So after I checked with a buddy of mine that works for Garrett NZ about my initial estimations, he agreed that they seemed overly harsh, with respect to airflow requirements.
With a couple of his suggestions, I have gone back to trying to make the 63r work.
Basically picked two targets, 320whp on pump, 400whp on E85. Do I see myself using E85, maybe. But, as has been pointed out to me, good future proofing. If I could confidently say NO to the E85 option, it would be best to get a gt2860rs over the gtx2863r. It is basically the same compressor map at moderate boost levels, but much cheaper. But the gtx2863r has much more ability in the higher boost range. Plus its billet wheel should help spool slightly.
Some points:
I chose to make target hp at 7000rpm as this seems to be where most fa20's are making it, give or take. Certainly not at redline. Most likely to do with stock head/cams, amd/or losing VE past 7000rpm.
I pumped up VE from BA's suggest of 0.90 to 0.95. Reasoning being that the motor is high compression, direct injection. Yada yada.
I have plotted down to 3000rpm. Now this DOESNT mean that the turbo will spool and make full boost at 3000rpm. I don't think it will. But, it shows that if the motor is able to get the turbo to spool this early, the compressor can support it.
Anyway, this is what I ended up with.
Looking at the results and I'm liking what I'm seeing.
The pump line is showing good efficency. Dropping the boost would result in the same or less efficiency as the islands narrow. The lines past the optimium island are also at about 45 degrees at this point, so less boost, less airflow, but you'll probably be in the same islands.
You could maybe go a little higher on the pump line, to 14 or 15psi. The islands get slightly wider and the past-optimium lines start to steepen. 17-18psi seems like it would place the line where the islands are at their widest, so I would estimate thats where the best efficiency would be found. However, I haven't plotted that, as I think the pump line as is, is pretty much where the knock threshold is (thereabouts) due to the high static compression.
The E85 line is there as sort of the limit as to how far I would push it. Looking at the map, the past optimium lines are near vertical, so reducing hp target, would lower the boost and airflow req'd and lower this line. It would bring the plots into better past-optimium zones, as again, they're near vertical at this point.
So after seeing these results, and getting reassurance that while these are still only estimates, they are most likely more accurate than my own initial estimations, I'm sticking with the gtx2863r.
Just quickly about the turbine housing, I'd already decided to go 0.86 instead of 0.64, purely based on the overwhelming preference of tuners already on this platform that 0.86 performs better. But my buddy pointed out that again this turbo is very similar to the gt2860rs (disco), and the same arguement over 0.86 and 0.64 has been debated to death when talking about the disco. He pointed out that comparing the two spooling on an sr20det, the 0.64 spooled sooner but slower, the 0.86 spooled later and faster. They both reach maximum boost at basically the same point. 0.86 will also cope with the higher rev range much better, and certainly with the e85 line.