What videoformat is your source media?
If it is in DV, here is an idea…
This is a shot in the wild, but I have noticed that – at least in certain situations – to go to uncompressed from DV _can_ have some advantages, when extreme re-rendering is needed. A blowup by 133% could be a situation like that. Because you don’t get excessive degradation in the process.
That does not mean that I’m not aware that DV is DV etc, but when applaying cruel changes to the source material, I have found that this route can give better results.
Another idea – taken from photoshop sessions (have not tested this!) is to uprez the image before blowing it up. Dunno what effect this would have in fcp, thoug. In PS, going from a bad jpeg, uprez’ing, creating a TIFF, Treating and then crop’ing is often a better idea than just blow’ing up the jpeg and applying changes.
This is defenitely in the 1 cent region of suggestions… -;)
Gunleik