Although, Hughes and Pelfrey are both coming off ~5 IP/start averages, which is less than the the Twins overall average last year. Even in his "peak months" last year, Pelfrey was under 6 IP/start.
6 IP/start seems like the top end for these four guys, which is what it would take to throw just 100 more IP than last year's starters. And that's when they are all pitching well. If any one of them has an off year, they likely will fall short of that level of IP improvement.
I think we can expect to see an improvement from Hughes. Pelfrey, maybe not so much. Overall, the net gain should still be significant, even if it's only 50-60 innings. That's basically one season of the worst reliever in the bullpen.