The stock shocks are fine and more than adequate and the correct lengths. They also don't seem to leak.
You would be better off spending a few hours tuning your shocks for where you run or what you do and learning what you have to work with already. Big bores would be a waist of money for the mini 8 imo and I buy everything for mine. If you buy a set of aftermaket shocks you will need to do all tuning to even achive as good of setup as what the buggy comes with stock. That means selecting the right springs, pistons, fluid and mounting etc... you might get lucky the first time, but I doubt it.