If you are caching to the disk, I would think that there would be something in your “wp-content/w3tc/dbcache/” folder. Check it out. If there is nothing in the directory, I would assume that the caching isn’t working.
I don’t rely too much on the info written at the bottom of my pages. It isn’t very clear messaging. Also, I had issues with my caching while I had the debug info on. Without the debug data my page loads were 100ms and with them it was 900ms.
I wouldn’t be too worried about caching your DB to the disk (slow anyways) as long as you have your page caching using enhanced disk and minify working properly. Those will be your biggest speed improvements.
If you could get a CDN up, that would be better too.
Note: if you find that you have times where it suddenly drags, minify uses a good amount of processing time. You might be better off with just combining the files instead. Another option is to manually minify the files yourself and using the combine only option. Of course, you would have to reminify them yourself each time the code changed though.
