https://spl-int.com/yeast-products/high ... rbo-yeast/The Saccharomyces cerevisiae wine strain in this Turbo Yeast is ethanol tolerant up to 18% ABV and capable of fermenting between 10—35°C liquid temperature (note: ethanol tolerance is significantly reduced above 26°C).
SPL Founder, Philip Jones first developed turbo yeasts and brought them to market over 20 years ago. Turbo Yeast is a range of fast fermenting yeasts combined with unique nutrition complexes for use in the production of alcoholic beverages.
They can be used to produce 96% potable alcohol, or can be used to create spirits, liqueurs or various flavoured alcoholic beverages (FABs) and have proved popular in creating products for the RTD category.
Combined with complete nutrition, our High Alcohol Turbo Yeast range contains products suited to various fermentation conditions, facilities, sugar sources and ABV requirements; no matter your needs, we’re confident we have a suitable product for you.
So according to this, with temps between 10-26 C and the most tolerant yeast on the market you can ferment to a maximum of 18% alcohol.
If you want to increase the strength further you could try a little freeze distilling.
Water freezes at a higher temp' than alcohol so if you put it in your freezer for a few hours you can remove the ice which will be water and increase the % of what you leave behind.