Accuracy
Don't want to start a huge "accuracy" discusion, but...
a Nitro tank and reg will increase the velocity consistency which will in turn result in increased "up/down" accuracy. But yes paint/barrel match is the only way to improve left/right accuracy.
Pirhana's are nice markers, I have only used a site marker version on Air, and while not top flite, I have never had one stop working except as a result of low gas. I played with it at Series 2001 tournaments, and while is had many problems (low accuracy, rof etc) many of them will not be a feature of a top end Pirahna model. They are so reliable that it is the marker I am plannign to get as my backup (when I have enough money...) .I own a cocker now (which I bought second hand) and I have had all sorts of problems with it (though I believe that all of that is fixed) and it is sweet so a cocler is a good marker to upgrade to, but to be honest a decent pirhana with a good set of barrels etc should serve you very well.
So I agree with KO, stick with the Pirhana, after all you can hang onto the Air system when you upgrade (so you may consider getting a better one on the grounds that you won't need to buy one for your next marker). And of ourse you may consider hanging onto the Pirhana as a backup should you ever have a problem (not that a new cocker should give you problems, its just that 10 seconds from game on and your marker isn't working for some reason sucks, just picking up a backup and playing makes life a LOT easier).
Richard
P.s. apologies for the many wrong spelling of Pirhana above, I don't really know how its spelt and can't really be bothered to check/change them all.