On Xbox360 it makes a pretty big difference, overall switching from List<T> to FixedArray8<T> gave us about a 20% performance boost if I recall correctly.
This is mainly because the Xbox360 has a really bad version of the CLR in terms of garbage collection, so reducing garbage gave us huge perf wins.
Raising the limit should be trivial if you really need to (keep in mind that a Body can be made up of multiple fixtures, each with 8 vertices). Just bump the setting which is set to 8 and switch references to FixedArray8<T> to List<T> or
make FixedArrayX<T> for the size you want. For sizes greater than 8 you will likely start to run into CPU overhead of copying huge structs around so switching to List<T> and incurring the garbage might actually be better.
If you are only planning on using this for Windows then switching to List<T> is the obvious choice, the CLR on Windows is generational and can handle tons of grabage per second unlike the Xbox 360 (also theh CPU bound nature of the engine means that
most PC's will simply process a frame faster).