i'd put my money on the BOV needing adjusting.
stalling issues and sluggish acceleration can be due to the spring being too soft. when it vents, the bov is staying open for too long = stall. if you are giving the car shit, the bov may open = crappier acceleration.
not sure about your fuel economy either .. i get just over 12L/100kms, calculated by distance travelled and how many litres it takes to fill up (the only real way to measure economy).
I don't see how an atmo BOV can affect fuel economy. the side effect of an atmo BOV is that the car runs rich due to the air escaping rather than being plumbed back in. But if you were running a plumbback, you would be still using the same amount of fuel .. just not running rich as all the air is still available?