I thought it would be interesting to check out Nathan's stats broken down by number of IP, so here they are (courtesy of Retrosheet and a little tinkering with R). I've lumped all Nathan's four full seasons (2004-2007) as a Twin together in the following.
In his first four seasons with the Twins, Nathan has never pitched more than 2.0 innings and he has never had an appearance in which he failed to record an out.
Note: "SO%" and "BB%" mean strikeouts per batter faced and walks per batter faced, respectively.
Update 08/07/2008 04:52 ET: Added ERA column.
Overall pitching line as a Twin
When pitching less than one inning
When pitching precisely one inning
When pitching more than one inning
First of all: Gardy has experimented very little with Nathan's role. 86% of his appearances as a Twin have been of the 3-outs variety.
Obviously, his numbers when pitching less than one inning are pretty horrible due to the fact that those appearances are mostly ones in which he has struggled (we can see that a sudden lack of control has probably been the main culprit in these situations).
The real lesson learned here is that Nathan pitches almost equally well when being "stretched out" as he does when he simply goes one inning. When pitching one inning Nathan has faced an average of 3.8 batters; that average is 6.9 when pitching more than one inning. Thus, it seems that Nathan can actually handle double his usual workload without experiencing a drop in performance. Nathan is on pace to log only 66.1 IP this season, which is about 4.2 IP less than his average totals from 2004-2007.
Now, the sample size for the "more than one inning" numbers is pretty small, so this is not a definitive result. A more thorough study would also look at Nathan's performance when entering the game with runners on base, and maybe adjust the numbers according to the leverage index.
But I'm still pretty certain that Gardy's decision to let Nathan handle longer outings in the future is a very good one - and not only because the rest of the Twins bullpen consists of little schoolgirls.