The 18% is what is rated for. Sometimes it can go higher than the rating. Pacman yeast has an alcohol tolerance of 12%, yet my braggot that was fermented with Pacman yeast is at 14% and still climbing. So trying to use the alcohol tolerance of the yeast to stop fermentation can be hit or miss. As DoctorCAD had stated, young wine tastes like a$$, and with the higher percentage of alcohol, the longer it has to age before it stops tasting like a$$.
Nobody here can really tell you what you can or can't do, so if you want to make that wine 18+% alcohol, I say go for it. But just warning you, it will taste like Satan's anus for a while.
I would probably suggest making the original recipe as it is first, and then maybe make a second batch of the Satan's anus wine, just for comparative purposes.
"Stop giving me credit for those stupid quotes! For that last time it wasn't me! STFU NOOB!" <-- Benjamin Franklin