I've heard of people getting 640 at the wheels with the normal Paxton kit, a tune and headers. Beyond that, you're talking about upping the boost (stock kit is only 7psi.), installing a new fuel system (bigger injectors with fuel rails, fuel pump, dunno if there's more), and yeah, probably AEM or SCT in the least.
My understanding is that $15,000 will get you 1000rwhp using the Paxton kit as a base from Underground Racing. Not bad considering the initial cost of the Paxton kit is quite high, and there is a lot of work to install it. If I didn't live in California and need to have the car smogged, I might have gone down that road. But the stock Paxton kit is CARB legal, and for that, I'm super grateful.
DC Performance just started using the SCT to tune all of their supercharged vipers. Sounds like a great system. Again, if I go to have mine tuned, that's where I'll go. Dunno how much to expect out of the tune. Probably worth a phone call to find out. The car just runs so good right now...
A few other items worth noting. Doug Levin sells a larger heat exchanger and better flowing water pump. The Paxton kit is o.k. for some extra ooomf on the street, but it does heat soak really quickly. Last week I made two dyno pulls, the second one made 30 hp less just because of the heat soak. I'm running a ScangaugeII which allows me to monitor the air intake temps. They'll drop on the freeway, but climb quite high around town.
I'll be upgrading the heat exchanger on mine soon and will be able to see if/how much it affects the air temps. I'm going with the water pump from the Ford Lightning. It's a reliable, high flowing pump and only runs about $90 on ebay. I'm also going with an 03 Cobra heat exchanger with two 6" pusher fans for extra cooling at slow speeds. Should be a nice set up. The heat exchanger and pump are on their way, but the fans are on back order for a few more weeks. I'll post the results when I get it all done.
On the blower whistle, you know, on my Cobra, I had the Vortech SQ drive (quiet version). You couldn't ever hear it. Now after driving around for a few weeks with the Novi2000, I feel like I really missed out on those great sounds back then. I absolutely love the sound!