To start, I have an 04 GTO, t56. Stock bottom end, MSD intake, mild cam and a single passenger front mount turbo setup (driverside exhaust feeds into passenger side manifold which feeds turbo), 8psi. I had the car professionally tuned (Speed Density so no MAF) in May 2016 and it drove and ran perfect since. However for the last month or 2 I noticed changes.

1) My AFR would normally side right around 14.6 at idle and cruise, now at idle it sits around 13.5 and often times it seems to dip to the 11-12s and searches around a lot. It still sits around 14.6 at cruise.
2) Normally when I get off the throttle to cruise to stop at a red light for instance the AFR peak to the lean side momentarily and then returns to normal, which makes sense to me being that I am cutting fuel demand, now when I let off the throttle it peaks to the rich side, I'm talking 10-12s AFRs and stays there for a bit until I stop or hit the throttle again.
3) When I WOT my AFR seems sluggish to drop to the rich side like it normally would (normal AFR for my tune was around 11.5 for the boost), now it seems to hang up around the 14.6 around and then drops or acts like it doesn't know what it wants to do, and when I do get them to drop to what I feel is a safe level to continue WOT (for troubleshooting reasons) the car seems to act as though it's hitting the rev limiter around 5500 RPM and stutters there. Feels as though it hits a speed limiter really.

I have no DTC's and I have checked various sensors, 02s, tps, MAP, checked for vaccum leaks (my boost gauge shows a constant -14-15 vacuum at idle which is pretty much where it was before), I was thinking of swapping the IAC. I pulled spark plugs and they look pretty darn good, better than I expected, lol, no oil or signs of excessive blowby, gaps still good. I also connected this handheld FAST tuner I have because it allows me to view some eng variables. So I know my tps, 02s, water temp, IAT, MAP, injectors are working because I can see the corresponding value changes, while I am no tuner I know they are the same as they were prior to the issue. HOWEVER, what I have noticed is that the scanner is showing cal. ENG LOAD at .7%. Constantly. Whether I'm idle, cruise or WOT the ENG LOAD value does not change. When i monitors this before a few months ago it showed change with actual demand. Any ideas on what this could be or what else I could check out to possible causes, I may have a pinched knock sensor harness but i didn't think that would affect changes at idle so i ruled it out. I also replaced the intake gaskets and no effect. Sorry this was so long but I was trying to be thorough. I'll try to post a link to some videos of what its doing when I can.

 

Short video of the car idling, I blip the throttle a little at the end

 

You should try new primary O2 sensors.

 

If it ran good for a while then started acting up, then the tune is not the issue.
The tune does not change and even altitude/weather variation will not make it change that much, so it must be something else.