The ONLY car I've ever driven where it was beneficial to downshift was my 2001 mustang cobra.

The 32valve motors are anemic below 4k rpms. That coupled with the fact that mustangs have to crawl through turns created a situation where downshifting didn't cost me any time. Essentially, I was going so slow, I had plenty of time to downshift in the turn before I would have been on the power if I hadn't downshifted.

You're probably going fast enough through the turn that the downshift is costing you time. I highly doubt that the power from teh additional revs will make up for the lost time from the downshift.