Poking around on northstarperformance.com, I noticed that they recently created a page for tuning the Bonneville GXP ECM's:
http://www.northstarperformance.com/gxptune2019.php
Sounds like its a $100 deposit, with $449 + $75 core charge for a stage 3 tune. They list their tuning stages here:
http://www.northstarperformance.com/tuning.php
This is the first mail-order tuning option that I have seen since MadTuner, which has since stopped offering tuning services. They claim 57 horsepower off of their stage 3 tune, not sure if this has been updated to reflect the GXP tune or not. Either way, thought I would share this here since it seems to be a new development in the land of Bonneville tuning!

