Assuming that a FF. . .
Sigma already make a 24-70mm f2.8 EX lens in Pentax fit ( I have the Nikon version at work and keep meaning to pick up a Pentax one ; only thing against it is it is a big heavy lump of a thing ) .
i'm sure a lot of my SMC-M glass will do very well , after years of shooting colour slides and projecting to large sizes , I think resolution will be more than good enough .
Pentax have made some excellent lenses in their own right , but also have history of collaborating with Carl Zeiss for some designs - you don't get much better than that .
With kind regards