Twinscroll worth it?
 

With a narrower powerboats than optimal I was quite pleased with the 6sp+4.875 combo at the last race (effectively very tight ratios and small drops).
4-5-6 around the whole track (no gears to strip in 5th).
Shifting at 7.5k btw.

1st is a nice trailer gear. :)

Yes, it is oversimplified. However what are you guys considering way down in RPM range here? I am not sure if what you guys are describing really needs to be fixed with the rear-end ratio rather than the transmission. Otherwise realistically the best 5sp option would be something without an overdrive.

Racing RX-7's, if the car is non-turbo, nobody touches the turbo transmissions because of the tall gears and the uber-lack of torque. But with turbo, the torque is not as much of an issue.

After I built the motor the 2560 wasn't enough. So after a lot of research I decided on using the MHI Twin Scroll TD05HR-16G6 10.5T. Reasonable priced, rebuild costs aren't bad. I been super happy with this choice. The spool up is great. (I want your spool data post #219)
That said, the header was a challenge to fabricate, so I would vote yes to a cast manifold for others to try this segment of turbos.
I don't know the answer to this but since most people start with stock motors would this flange fit other MHI turbos so as you upgrade the motor you could still keep the manifold and choose a higher flow turbo as necessary. Also the exhaust spacing between Mitsubishi and the BP4W head is very similar so maybe there are two potential markets here.
For what it's worth, here's the link to my set up:
https://www.miataturbo.net/diy-turbo...o-build-79259/
Cheers,
-JB