I don't think it would affect perfomance noticeably. The control name lookup can be made very fast by storing names in a hash table. Besides, any internal search/lookup method in mirc is much faster compared to scripting. For example, I would expect that the mere evaluation of $dname in something like /did $dname blah blah is slower than looking up the ID for the "blah" control internally. In any case, if one noticed a decrease in perfomance, they can always switch to the old method with numbers.
Named controls would be handy for a lot of people, including myself.
