You do realize that all commands are sent to the server by mIRC as raw commands don't you? There is no special mIRC syntax for the server.
Back when everyone was using 80286-80486 machines and perhaps even with p1-p3, this simple evaluation might have made a difference that you could somehow measure. It doesn't anymore.
I've tested it with identical p4 3.00 ghz machines with no difference found.
It is only a different evaluation made by YOUR computer. On the server end, where more evaluation could effect the efficiency of the script, there is no difference at all.
/mode # & /raw -q mode # are both evaluated by mIRC and then sent to the server as the identical command.
The only reason to ever send a raw command to the server now a days, is if there is a new command that mIRC cannot understand.