No you can't downgrade from the leak without S-Off. You can however use the "old firmware" patch to run most older ROMs. I think Scott posted it in the dev forum. And FYI, this should be posted in Q&A or General. The dev section is for posting ROMs/Mod/etc.. I'm sure a mod will move it shortly.
“Around here, however, we don't look backwards for very long.
We keep moving forward, opening up new doors and doing new things,
because we're curious...and curiosity keeps leading us down new paths.”