When you're refering to laying a 50 Ohm trace, you're referring to the SMA input and not the interface between the output of the Op-AMP(THS4508) buffer
and the inputs of the DRS4(IN0-IN8). Is there a recommended diffential impedance for IN0-IN8?
The requiremnet is the same as for any high speed analog board, there is othing special with the DRS4. If you want to terminate your line with 50 Ohms and you want a matched impedance layout, you route all lines with 50 Ohms impedance. Truth is however that nothing is perfect. The SMA connector is not exactly 50 Ohm, the PCB gets a 10-20% variation depending on the manufacturer. So even if you try hard, you will never have a 50 Ohm matched impedance. On the evaluation board we made some compromises as you have seen, but for us the board works satisfactory even with this compromises, and you can test it yourself with real hardware (namely the evaluation board). If you can do a better job, try it. But usually these compromises have only little influence on the signal quality.
I'm an engineer at UCLA developing a board with the DRS4 chip. Our team has a question on what might be the required trace impedence for the analog inputs. Can that information be provided?