just an update on this... looks like have finally sorted the issue with the car constantly going into limp mode, SCV was the culprit, since replacing it over 3-4 weeks ago, haven't had that issue come up at all, slowly drivability and mpg has improved a lot as well, since then got a carbon vac clean, and took it to Nissan how did a pump down on the fuel pump, it was showing low pressure...
since then car is running miles better, day and light difference from first getting the car, overall getting around 70 - 80 miles more per tank...
although it is a lot better then before I would still say it isn't perfect, as feel should get better mpg then I am...