RX8Club.com - Second Opinion(s) on Data Log - Primary O2 Sensor Health & Lambda

I'm checking everything possible to try and resolve the persistently elevated long term fuel trims I'm having. An independent shop found no vacuum leaks, and the dealer gave the car a clean bill of health as well. I'm struggling to find anything that would explain why my MAF reading low, and has started to read even lower recently (down from 4.5 to 4.3 grams/second with the occasional dip to 3.9 grams/second which is accompanied by a "cough" from the engine).

I followed the procedure outlined in the workshop manual to check on the health of my primary O2 sensor. I logged vehicle speed, engine RPM and primary O2 sensor current as the car is driven at >3,000 rpms and then allowed to decelerate. The manual said nothing with regards to keeping the car in gear or not, so I did runs with it in gear and ones when I shifted into neutral - no substantial difference other than the rate at which the vehicle's speed decreased. In both cases the O2 sensor's current meets the specification of increasing to > +0.25 mA; specifically, it increased to and stayed at +2 mA.

I decided to go one step further to see if I could glean any additional info. So I logged a run at full throttle (attached picture). In this log, I shifted from 1st to 2nd gear at 6ish seconds, and then held the throttle all the way to the floor for the entire run up to 8,000 rpms before letting off the throttle and going into neutral.

I'm new to data logging, but to me the following things seem strange:
1) The decrease in the slope of the trace of RPMs at the top end (around 7000-8000 rpms).
2) The slow of the RPMs as they decrease, it's rapid at first and then slows down substantially as it nears ~2000 RPMs (starting at approximately 17 seconds). Seems like a slow return to idle, but no codes.

Any advice on where to look next or what other conditions I should obtain logs under are greatly appreciated. Other than replacing the MAF I'm stumped (for now).