Okay, here's the update on the Tamiya visor: mine arrived from Japan yesterday and I love it. The Optivisor is getting returned to Amazon.
The Tamiya Magnifying Visor comes with three different, snap-in lenses:
1.7x magnification for a 25cm focus distance
2x magnification for a 20cm focus distance
2.5x magnification for a 15cm focus distance
There are apparently two optional lenses in the form of snap-in monocle:
3x magnification for a 10cm focus distance
4x magnification for a 7cm focus distance
The Tamiya product is very comfortable to wear, and the lenses seem to me to have a wider field of view than the Optivisor. Each lens has its own color coded storage sleeve, and the whole visor has storage pouch as well. This is a really nice product, with great versatility. I highly recommend it.
Take advantage of the strong dollar vs weak yen now. Shop around on eBay and Amazon, and get it straight from Japan. Sellers in the U.S. have probably purchased the product at earlier, strong-yen prices (and are trying to sell it accordingly to recoup their investment). The Japanese strategy of weakening its currency to spur exports is working on me.