hey guys, i did a little more tinkering and decided to swap out the wastegate actuators, swapping back to the "medium boost" actuator, and the overboost problem is completely fixed.
i have no idea why the problem existed with only the low boost actuator, but swapping it out completely fixed the problem.
and sorry, i've been on an emotional rollercoaster over this the last 2 days
so i dyno tuned today, and the short story is that this turbo is awesome. running the same boost as my previous 6262 setup, it made 12psi 1500rpms sooner, and made the exact same power. here are the graphs below (scales are different, and different software, but you can see the difference in response)
so the result was 338whp @ 12psi with very good boost response. enjoy the pics and videos.
previous 6262 setup
previous 6262 setup boost response
new EFR 8374 setup
new EFR 8374 setup with power and boost plot (notice the boost is in ~1500 rpms SOONER)
hot exhaust
RX7 dyno, EFR 8374 IWG turbo, 338whp@12psi - YouTube
RX7 BW IWG EFR8374 dyno hot exhaust - YouTube
RX7 BW IWG EFR8374 In Car 2 of 2 - YouTube