Tuning with a DPF

Is it harder to tune a vehicle with a DPF, I'm thinking because you would not see smoke or would an opacity meter work on the tail pipe?

Correct, tuning a vehicle with a DPF does present some complexities. The best option is to use an opacity meter pre DPF which in itself is a fair amount of work. The more likely solution is to use a lambda sensor and set a minimum lambda limit which will result in minimal smoke - typically in the range of 1.10 lambda or thereabouts. If you tune richer then you can make more power but you risk filling the DPF quickly and you'll be doing constant regens.