If you need two or three masters on the weight axis depends on the design of the font. So you can try to us only two, interpolate the instances and see if you like the result. If not, you add add the Regular in the middle. If you need corrections only on some glyphs, you can add add extra 'masters' per glyph (in Glyphs.app with a brace layer and with an extra UFO in designspace files, I believe). That way you save a lot work.
The problem is that there is not one Windows Cleartype. The result looks a bit different in each browser. So you need to check each browser/app in a real environment and even on different versions of Windows.
There was a problem in Adobe apps (maybe CS6) with UPM higher than ~4500. When you convert to outline it smashed all nodes higher than that down. Printing and pdf export was fine. I didn't test this recently.