I can't attest to how it will run with NO exhaust, but I can tell you how it ran for me with my straight through exhaust.
I noticed a slight gain in power, and no pinging or lean spots (significant anyways) in the RPM range at all, but I never really got on the bike at all because I knew it would run lean at WOT. If you run it with no exhaust, just go easy on it, and you'll be OK until you can at the very least shim your needles.
After I shimmed my needles, I completely eliminated the lean spot between 2 and 4k RPM's. I can only imagine what my bike will handle like once I get the jet kit for the bike with the new K&N and my straight through exhaust.
And yeah, it's RIDICULOUSLY loud...