The -portable switch will work as advertised :-) It will make mIRC avoid use of the registry. However that has nothing to do with folder locations.
In the next version, if you want to make mIRC avoid the registry and to use the executable folder for storage, you would specify the -portable and -r switches (I've modified the -r behaviour in the next version so that -r without a path will default to the executable folder).
If you don't specify -r, then mIRC will check if mirc.ini exists in the executable folder. If it does exist, it will use it. If it doesn't exist, mIRC will use your Application Data/mIRC folder for all storage.
That said, I may change -portable so that it 1) avoids the registry and 2) uses the executable folder. I would then need to add a new command line switch -noreg to avoid use of registry separately for users who want that.