It won't quiet double it, and you'll have charging considerations, series + parallel packs need to be balanced every now and then moreso than a simple series pack. If you don't change the charger to accommodate the life of the pack at least as far as the number of charge/discharge cycles go will probably go down a little bit, also because two cells never match, let alone three in series let alone three series two parallel you'll probably increase the zero power drain of the batteries as the stronger of the batteries tries to charge the weaker one's and the cells internal resistances leach out just a little bit more power when not being used.
No thought given to a Lithium pack? Least as long as you can match the voltage you need.