Question - Is there a way to get Lightroom to discard all of its previews and then rebuild them?
Go into the catalog folder with Lightroom not running and take the Previews.lrdata out of the folder...when you launch that catalog, it'll go through and build new previews from scratch based on your preview settings prefs. If you are sure you want to completely rebuild the previews, you can trash the file instead of moving it out.
You can then decide which images or folders of images you want to build 1:1 previews for. I build previews for folders of images I want to work on for selection editing, keywording, etc. The 1:1 previews don't help with Develop...increasing the Camera Raw Cache can. Currenlty you can now set the ACR Cache to 200gigs if you have plenty of drive space...