I have been involved in “the world of wine yeast” since 1991 and the highest alcohol produced by a yeast during fermentation that I have ever encountered during all these years is 18.2%. I have however read that 18.6% has been observed in Japanese saké wine. During the past southern hemisphere harvest in the South African wine industry we experienced a massive heat wave in the first half of March which lasted a week and was disastrous for wine production. Cellars could not pick the grapes fast enough, and dehydration caused sugars to skyrocket. One poor chap phoned me requesting advice on how to ferment his 32 °Brix Cabernet Sauvignon. Since water addition is illegal in South Africa I suggested Anchor VIN 13 at 40 g/hl, Lallemand Go-Ferm Protect at the recommended dosage, a fermentation temperature of 23°C, the judicious use of complex yeast nutrients and prayer. The winemaker phoned me yesterday to report that the wine’s residual sugar is now indeed at 17 g/l and the alcohol at a staggering 19%! And the sugar is still dropping! This is the highest alcohol produced by wine yeast during fermentation of grape juice that I have personally ever encountered. I would be very curious to know if anyone out there has encountered something similar or even higher?
So, it seems the recipe for ludicrous sugars in the absence of the black snake (yellow snake…depending on the colour of the resident cellar hose) is VIN 13 wine yeast at 40 g/hl or more, Go-Ferm Protect, a low fermentation temperature to minimise alcohol toxicity, everything short of gold and diamonds in terms of yeast nutrition and faith in abundance. 
VIN 13 – the first hybrid wine yeast patented and commercialised by Anchor Wine Yeast in 1993. It is a hybrid between S. cerevisiae subspecie cerevisiae and S. cerevisiae subspecie bayanus. It has the fermentation capability (and more it seems) of “bayanus” yeasts and the aromatic capability of “cerevisiae” yeasts.
Go-ferm Protect – a yeast rehydration protectant patented and commercialised by Lallemand. It is an inactivated yeast based product very high in sterols and long chain fatty acids for the purpose of enhancing alcohol tolerance in live yeast cells.