Putting in a check to prevent NaN input values seems to have cleared up the issue now, and it would also seem my Kp is just that high...Im up around 500 now and finally in the ballpark.
@jremington: Should it become necessary later, and because it sounds like a useful tidbit, how would one go about adding new print statements to the library? Is it just a matter of opening a file in the library and inserting print statements (I've not ever dug into a library, just added them with the "manage libraries" feature in the IDE and ran...)? Will they print straight to the console without any call or extra code from the main loop?