Holic,
I believe that Cathay had a similar incident a few years back on a 747-200 ( gear-box caught fire and the two shots failed to extinguish it). As you point out, by the time you have completed the fire drill correctly and determined that the fire has not extinguished, you will be almost through the flap retraction and may be at approx. 2000 feet. Descending to circling minima will not help if the weather is at landing minimums!.....rather having declared a Mayday and quickly sorted through the appropriate checks and c/c brief + short PA, aim to couple up with the ILS and carry out an auto-approach to a manual landing ( in accordance with your company's SOP's, of course). By the way, the 747 classic would cope with a quick return, as long as you carefully monitor the automatics and be prepared to go manual to capture the localiser/glideslope. For the 744, no problem -manual landing with Flaps 25 and the Vref close to the departure V2.
Good luck - don't forget the aim of the game is a safe conclusion!