Hey skep18, its hebs from msioc. I saw you asking about this in the shoutbox earlier and I answered you but I think you are gone. For some reason I decided to check out the cobb section tonight and saw your post so here is my response.
OTS maps are built to have some conservatism but they are somewhat of a shot in the dark. Things vary from car to car, such as maf scalings (which controls how accurate your afr's are), and wastegate duty cycles (which controls how accurate your boost is). I have run some absolutely amazing base/ots maps but not a single one was %100 perfect for my car "off the shelf". The meat of the map is what the tuner is telling the car to do (how much timing to run, how much boost to run, how much fuel to run, etc...). But other tables usually need tweaking to get the car to do exactly what the car is supposed to do.
When you get a custom tune, not only will the car do exactly what the tuner designed the rom to do, but he can be more aggressive because of it. With most OTS maps, they have to build in some conservatism to make it work for the majority. However this usually means uber rich afr's which can foul plugs and sensors, or lower boost and timing which makes you slow
Also, in the first post you forgot to mention who is coming from CO to tune your car. We are having Eric Minehart from http://www.torquedperformance.com
fly in for a group tune. I believe that anyone who has heard of Eric would agree that $250 for him to fly half way accross the US to tune your car, and future support, is a pretty damn good deal.