I don't think there's a reason to bother with an OEM ecu ever again when the termi x is so cheap and easy to use. I loaded a base map that I built up using some best guesses and have just been driving the car since then while the fuel auto learns. There are some manual tweaks to do to improve driveability/idle/coldstart etc, but literally the car fired up and was ready to go WOT with the base tune I built.
It feels much faster in control of the engine. The timing and fuel control is extremely fast and it translates to a smoother running engine. You don't feel that slam into PE mode for example. I had an early Gen4 ecu in this car with the E40. The Gen3 0411 stuff was painfully slow and low resolution imo and the difference would only be more noticeable.
You can command whatever AFR you want at any point - I'm idling at 14:1 because my cam is pretty big now, cruise at stoich, and as RPM/load climbs it progressively gets richer up to 12.5:1 at full power.
The ECU self learns quickly and accurately based on the wideband improving your tune just by driving around. As your closed loop correction shrinks you tighten down the learn window so erroneous data can't blow up your nicely dialed in fuel map. In the WOT region I can probably already narrow the learn window to +/- 3% and still allow closed loop to be more aggressive, but it won't make large changes to the base map.
Closed loop wideband operation at all times like a modern car is so nice to have finally.
Real time tuning while the engine is running if you want to tweak things.
Custom tables and I/O's for adding things that dickheads want like 2steps, no lift shift, flex fuel, power adders etc. You aren't stuck forcing the factory ECU to do something it doesn't want, or running standalone systems without ECU integration. I still have to add these things in I was waiting for some harness stuff and also just taking baby steps sorting the car out. I'm excited to play with e85 and NLS though.
HPtuners is good and has only gotten better, but you're still limited by the hardware. Also kind of funny I'm replacing an ECU that was designed ~20 years ago at this point so I'd be disappointed if it wasn't a significant upgrade.