As title says, i'm interested if there are any cons of running the car in full time open loop ?
If yes, what are they

 

Only "con" I can think of is that you need to make sure your complete cruise/idle area of the fuel map is dialed in as you can't just rely on the front o2 to get you to stoich AFR.

 

^^this, very hard to dial in and very inconsistent from season to season.

Also, EGR won't be activated on cruise, it may kills mpg even if you try to compensate the timing advance in the main map. Recirculated gases raise the combustion chamber temperatures making effective an increased timing advance.

I recently switched to mrfred wideband closed loop AFR control (9653) and absolutely love it. I target 15,6 and my AFR swing from 15,1 to 16,0. It will improve MPG for sure.

 

I have evo 4, that has no EGR hardware at all

 

Still pretty hard to dial and have a stable AFR. On MAF it's much more easy.

The inconsistence is much worse in SD when using the fuel temp sensor to read manifold temperature.

I did both and on SD I ended up switching it to closed loop (with wideband) to get much more stability. (post 240 for Evo 8)