So,finally, to answer your questions - I hope it's worth the wait
Jumps
For jumping, it is important that your car's driven wheels can spin up (increase in speed) when leaving the ramp. The motor will make less RPM under load, so if the car leaves the ramp, the motor will rev up if you hold the throttle. That is, unless you're suffering from wheelspin, or another reason why the motor is already at its maximum RPM.
This is not only a FWD handling trait, this counts for RWD and 4WD as well. However, the FWD being a car that more easily has wheelspin, it can be more pronounced if you don't take it into account. Also, as Kengor pointed out, don't be afraid to run a powerful motor in it, FWD is stable anyway
Shocks
The spacers are part of the geometry I developed for the FF210, and are used to achieve the right amount of droop depending on the type of Durango shocks on the car (The 210V1 has longer shocks than the 210V2). As this answer might suggest, running spacers inside the shocks very much depends on the design of the car.
Diff
Diffs are as much dependent on grip levels as on driver preference. Myself, I prefer ball diffs in any 2WD because they cope much better with changes in grip levels throughout the day (think of wet vs. dry conditions) and between events (clay vs. astro) than gear diffs do.
That said, to answer your question: I have run a gear diff in prototypes. It gives more drive as long as there is good grip; also, generally you can run the oil a little thicker than a RWD, but it's easy to overcook, hampering cornering and stability.