No guarantees about the quirks of your Garmin Edge 800 but it'll certainly reduce the frequency / likelihood of it crashing when navigating the GPX. Note it slims GPX tracks only, not GPX routes which are a different beast.
They aren't much different at all I don't think if you use point to point navigation, riding between marks of the turns.
I slim down GPX tracks/routes semi manually for use on an etrex.
Get gpsies.com to reduce the points automatically if needed (not really needed with my own routes from cycletravel as they only mark turns to begin with) then fine tune on the gpsies map, then semi automatically strip stuff out using a simple text editor.
There's an awful lot of not needed junk in most gpx files.