The mapping card should not slow down your 998c Si unit.
Biggest problems with switching the mapping card back and forth will be the possibility of dropping it like probucktails stated and having to get the card door resealed properly while on the water (and bent over at the bow).
I don’t know about installing/removing the map card while the unit is in operation. Personally I do not like to install/remove any memory cards while the device is operating (unless there is programming to support this like in your computer) but understand that many do so without problems.
