you can only remap cars with an ecu, divvies lol
diesels are similar to petrols, you can fit uprated camshafts, uprated exhaust systems etc etc.
A mechanical injection system you can just turn the fuel pump up and that will chuck more fuel in, but if you can't get it to burn efficiently then it may lose power.
The black smoke is because you have to inject more diesel than for optimum burn to get a bigger explosion, to get the turbo to spin up, to burn the extra fuel that you're chucking in, so you get black smoke till the turbo is up to speed.
As with any turbo car, decat(s) and free flowing exhaust systems give great gains, and contrary to what people think a diesel performance exhaust will sound just like a petrol exhaust note. Note than on cars with variable vane turbos running high boost(most modern high pressure injection cars) removing the cat can make the turbo spin far too fast and blow it up VERY quick. They have no wastegate to control boost unlike petrol cars, the fuelling controls boost, chuck more fuel in to make more boost.
There's a guy on briskoda getting over 260bhp out of his 1.9 tdi PD engine, with more torque than most supercars.