You'll need a downpipe with a high flow cat. It the boost quicker, and you'll notice a difference. The reason you go stage II with a downpipe, is to prevent boost spiking with the added flow. I'm happy with my tune. I'm not sure if it's worth $500 for the tune and however much you're going to spend on the downpipe. To save money, you should get a custom downpipe from a reputable exhaust shop, at least 2.5", preferably 3".
I also have a high flow DP/exhaust. I posted a poll about which mod should be next, a tune or a FMIC. The FMIC won overwhelmingly. The thinking is why get a tune if the ECM is going to retard the timing because of high inlet temps from the stock IC?
A tune is good if you don't see 90 degrees plus temps 3-5 months a year like we have here in SC. So for me FMIC will probably be next.