Bersaglieri, to answer your question, the reason I'd like to change out engines is that the small cam just isn't capable of the horse power because of the little amount of fuel that can be put to the engine.
The 855 is a low pressure common rail fuel system. The injection event is created by a lobe on the cam shaft which pushes a push rod that operates a rocker arm that causes the injector to fire. In these motors the barrel and plunger are inside of the injector and all the injection pressure is created by the injector. The small cams were have a smaller diameter cam shaft and are prone to premature failure due to wear and actually breaking the cam. If you look back through this thread, I put a new cam shaft in my engine. The hard surface had been worn off cylinders 3 and 4. Cylinder 4 wasn't even firing and cylinder 3 must have been very weak.
The small cam also has a smaller cam profile than the big cams do. Therefore, less fuel can be run through a small cam fuel system than a big cam. The small cam, i believe, has 0.177" of injector travel where the big cam has 0.240" of injector travel.
Where a 3406 A, B or C uses an inline injection pump (I don't think I have to explain that). My father runs a 3406B, which hangs with the KTA, V8 Mack and Electronic Cat in our club. There is just more power potential in a 3406 than in my small cam. If I can get between 500-600 hp to the ground with mine some day, I would be super happy.
The only real reason that I have to go to a Caterpillar vs a Big Cam is just biased opinion. I think a Big Cam can make relatively the same power as a 3406 B or C, with some minor engine work (injectors, fuel pump and turbo), but its Cat vs Cummins.
With that I cannot fault this engine what so ever. I think it either really likes me and is thanking me, but it has done great for what it is. If it was 400 hp to the ground right now I'd be happy. But it runs like a sewing machine since i've had the pump rebuilt, injectors rebuilt and the new cam shaft. So I would not steer anyone away from an 855, depending on their goals.
I actually was against getting anything that didn't have a Caterpillar in it when I bought this truck, but it came along at the right time at just the right price. I have always wanted an A model Kenworth and this one just fell into place. There was some consideration put into selling it to buy something with a Cat or an electric motor, but I had just put too much time and effort into this truck to sell it. Plus I love it to much to sell it. Maybe some day it'll get a Caterpillar put in it, but nothing in the near distant future. I want to make 600 hp with this motor before I think about changing it out.