Carson Cistulli has a great graph showing the relationship between fastball velocity and xFIP (from 2002-2011). Click the photo, but it basically works out to:
>= 95mph - 3.10 xFIP
94.0-94.9mph - 3.32
93.0-93.9 - 3.49
92.0-92.9 - 3.64
91.0-91.9 - 3.84
90.0-90.9 - 4.02
89.0-89.9 - 4.24
88.0-88.9 - 4.33
87.0-87.9 - 4.33
86.0-86.9 - 4.38
85.0-85.9 - 4.46
<85mph - 4.40
Where do the Astros fit in to this?
Well, let's go about it this way, looking at the starters (stats via FanGraphs):
Bud Norris (2009-11) has an avg 93.2mph fastball velocity, and a 3.90 xFIP.
Wandy Rodriguez (2005-11), avg 89.4mph fastball, 3.93 xFIP.
Brett Myers (2002-11), avg 90.2mph fastball, 3.86 xFIP.
J.A. Happ (2007-11), avg 89.8mph fastball, 4.54 xFIP.
Jordan Lyles (2011), avg 89.8mph fastball, 4.13 xFIP.
First of all, it's disconcerting that Bud Norris is the only starter from 2011 who has a fastball above 91mph. Lyles is right about where he historically should be, Wandy and Myers fared better than normal, and Happ and Norris fared worse. Do with this what you will...