Of course they mix, they have too, they use many of the same parts. The difference is the commercial adheres to the EPA/OEM specs for replacement parts.
Performance tries to go beyond OEM specs in order to draw the most out of the engine. So the performance tech will use many different parts and customized settings to gain HP and torque.
The commercial end of the business keeps most shops busy with maintenance for local municipalities while filling in with performance work.
The need is there for both however the conventions for both divisions are separate. I'm sorry I like to educate rather than ignore when attempting to change a view (smoke)!
The commercial end ignores the performance side for that reason in order to not get their government agreements reviewed because of the smoke in performance.
In the meanwhile a few guys in performance are making great gains in producing HP without the smoke. Also Diesel Motorsports is encouraging cleaner tunes for performance, the techology and parts just are not quite there yet. But we are getting there and making great advancements.
Some of these competitors are pretty innovative in their ability tuning high performance diesel engines, it's impressive.
One day they will put their great minds together and we might catch up with the European market!