I don't know what those numbers came from, but according to Baseball Prospectus' Park Factor, EnronMaid ranks 7th with a 1037 PF (3.7% increase in offense over an average park) and Yankee Stadium ranks 19th with a 977 (2.3% decrease from an average park). Baseball Reference uses a slightly different method, and has very similar results of 103 and 97. In total, this works out to about a quarter of a run per game. Of course, Clemens will be pitching in the NL (without facing a DH) and have a better defense behind him this year, so his ERA shouldn't be rising.

In the end, though, the park factor is largely irrelevant unless you're preparing for your fantasy season. Clemens helps a team just as much whether he's in a hitting park or a pitching park. If he's in a hitter's park, he'll give up a few more runs, but that will be balanced out by the offense scoring a few more; it doesn't make a difference in how many games the 'Stros win.

The more relevant issue (and the one initially raised by Saro) is whether the park is particularly poorly suited for a pitcher like Clemens. Ideally, you would look at park factors for a bunch of different stats (how much it increases/decreases HR, K, BB, 1Bs, extra base hits). Without doing that, we all seem to believe that Enron primarily increases homeruns, which would hurt pitchers who give up a lot of flyballs. As I said earlier, I don't think this is a huge problem for Clemens, without doing a lot of math, I'd guess he's around average. For Pettitte, on the other hand, this is a benefit because he's a groundball pitcher who gets a lot of Ks. So, he should be benefited by pitching in Enron (because he probably won't give up many more runs than in an average park, while his opposing pitchers will).
Of course, Pettitte was probably also slightly benefited by Yankee Stadium because he's a lefty. (I have no math to back me up on that).

OK, now that I've been writing this, I've become curious enough about Clemens to do the math. Out of the 92 pitchers who qualified for the ERA race last year, Clemens ranked 31 for the least flyballs per inning. Pettitte ranked 17th.
So, they are both well suited to pitching in Enron, particularly Pettitte.