that's a nasty idea, although in practice i've found I like to fire the terrorfex BEFORE i shot everything else. Yes that means I for-go the -1 for being under half strength but it USUALLY means i can fit more guys under the template, reducing they LD test by 4,5, or in rare cases 6. If you start killing guys to get it under strength your opponent will put 'holes' in his formation to get less guys under the template.
btw I use terrorfex a LOT. all my warrior squads, my hellions squads (move 12", then fire terrofex 12" for a total of 24",people hate that) and sometimes on my reavers. IMO it's an invauluable weapon in the DE army