The normal diametre for Wayfarer sheets is 8mm – genoa and main.
I think 10mm is too heavy, and although 10mm is kinder on the hands the sheets will not pay out so easily.
There are a couple of reasons for having the block on the boom inboard a bit; When using the boom crutch and the tent it keeps everything neatly inside the tent.
Gybing there is less of a tendency for the mainsheet to snag on the quarters.