The C-15 can easily be flashed to 550 hp. I don't know of anybody "custom" tuning the Cat motors. I believe that a Cat dealer is the only one that can do up the hp for you. The downside of that is they will charge you an arm and a leg to do it.
If Cat charges $2,500 more for 550 hp vs 430 hp, that is what they will bill you. The way the Cat tech explained it is their computers are linked to Caterpillar corporate. When corporate sees that they flashed the hp up, they bill the dealer that performed the work who will in turn bill you. I know this is the case when they are under warranty, it might be different if your warranty has expired.