at 300m there is no difference at all. Actually the patch will help you to set lower DP level for 300m (that is resolution 23 in Basecamp) while giving higher DP values for higher resolution. That came because for contourlines it needs very low DP value for resolution 23, while if you keep that low DP value for higher resolution, it makes the map unusable on device and even slow in Basecamp.
resolution 22=700m
resolution 21=1.5km
resolution 20=3km
resolution 19=7km
resolution 18=15km (if you use high DP filter - this is the first resolution where the simplification will mean the route is visually quite different from the underlying road, but still very easy to know which road it belongs to(
resolution 17=20km
resolution 16=50km
.....
with default style:
levels = 0:24, 1:22, 2:20, 3:18
overview-levels = 4:17, 5:16, 6:15, 7:14, 8:12
So 17 will never actually be shown on your device. That's why I mean - even giving high DP filter values (though in the branch because of less zig zag you achieve the same size with lower DP values!) will only be a bit out of place if you autoroute Berlin to Madrid.
Actually the basemap of most newer Garmin devices has the following resolutions:
level 0=17
level 1=15
level 2=13
level 3=12
level 4=11
level 5=10
level 6=9
it's too bad that there is no basemap starting with level 0=18
- I feel that would give better performance. resolution 18 is already quite slow to scroll around if you have topographic maps no matter how much you reduce the data - because it will need to read in quite a few tiles each time. But OSM data is not very suitable to create a new worldwide basemap.
The values that I mentioned DO of 8-10 actually seem to be lower or similar to what Garmin is using. The default of 2.6 is crazy detailled in comparison.