This American article from Car and Driver testing five different wheel/tyre sizes from 15s to 19s in a 2.5 Golf is from back in 2010 but I've not seen it referred to before. C&D have always produced fairly intelligent journalism so I trust their results.
It's interesting to see some of the effects that are often discussed here - a slight slowing of acceleration (not enough to feel, I would think), a surprisingly small improvement in braking performance and a whopping 10% loss of fuel economy going from 175 to 235 width tyres. The big surprise is that the 19s got slightly lower skidpan results than 18s - though there we do get the American obsession for measuring things with numbers, so it is steady grip they are measuring not the (more subjective) roadholding.
Effects of Upsized Wheels and Tires Tested 
