What is the fastest method to convert m2t files from
720 24p HDV m2t to DV Quicktime that can be imported
into a final cut pro timeline with renderless editing in DV?

I use streamclip after capturing HD100U Footage 720 24p in HDVXDV.
But, using streamclip, I can't seem to find the a workable setting
for the downconversion? Alway makes clips the Final Cut has
to render again for realtime playback...

One of the tricks with Streamclip if finding the proper settings. I had some problems when I first started using it when I'd try and export to a .dv. I dont have an HD100 but I know that Apple really likes quicktime. Try exporting as quicktime using the DVCProHD codec or whatever you're planning on using in the timeline set quality to best and frame rate to match your sequence. Good Luck.

what is streamclip?