Ok. So I rang the manufacturer of the BOV and explained the issue with the power-loss and delayed acceleration. Straight away, they said it could be a small spring.
A little history: When I had the BOV delivered, I had two additional "big" springs. These were RED and BLUE....
The manufacturer said they were sending me a "small" spring and I should replace the one inside the BOV with the one they were sending. Im still awaiting for that and should be with me within 24hours, meantime, I checked the strength of the springs I had, the blue one was "quite stiff" lol and the red one, was very stiff, so I proceeded to open up the BOV and take a peek.
There was a YELLOW spring, which was not very stiff at all, not compared to the BLUE or the RED one anyway. So I replaced the built-in YELLOW spring with the blue. Gave the car a little blast and found that acceleration was indeed improved, there was little lag, BUT I did get some backfiring from the exhaust - only a pop now and again. So when I got back to my driveway, whipped the BOV off, replaced the BLUE spring with the much harder RED one. Took it out again. WOW. Performance was MUCH better and the whoosh sound only came on when I got above 3,000 revs or so. NO LAG at all!!!!
So, next on the plan is: 100 mile journey tomorrow, hoping the lights will sort themselves out - I still got the occasional "pop" but I think it might be the ECU still configured to the weak yellow spring that came with the BOV.
When Ive done my 100mile journey, Im hoping the lights will disengage off the dashboard and I'll be a happy scooby owner once more. If not, Ill be taking it back to the "specialist" to get them to reset my ECU for free - they'd better had anyway, as I paid them to diagnose and repair the lights and all they done was repair the lights. He didnt even know I had a cold air induction kit fitted as he didnt lift the bonnet. NOT VERY GOOD AT ALL.... I may send something to Subaru to let them know that the "specialist" shouldnt be looking after their cars!
Im hoping, fingers crossed that tomorrow I will have the definitive end to this issue and that basically if anyone fits a BOV to their WRX 2009 and has major lag and fuel issues, it'll most likely be the spring in the BOV not being tense enough and that if they have spares given with the BOV, fit them before spending your hard earned cash on a specialist who might not have a clue!