stock heads are somewhere near 58-60cc depending on mfg tolerances. The x heads are the best of the gt-40 family of heads. They still come up a bit short when compared to AFR's, Trickflows, Canfield, Brodix. But they are good streetable heads.
The GT-40X heads were one of the best out of the box heads ... almost 15 years ago.
By today's standards and available choices of aftermarket heads, they are OK. The casting is a good quality, and you don't hear many complaints about their hardware (valves, etc), they just don't make as much power as other heads.
Close in price are Trick Flow Twisted Wedge and Edelbrock Performer heads. Most people would go for the Trick Flow heads.
If you are looking in terms of what they were designed for, then yes, they are excellent heads. They are designed for a street car to make good streetable power. They were built with reliability in mind, as are most Ford Racing parts.
Now if you are comparing them to other heads out there, say Edlebrock, Trick Flow, or AFR, that is kind of apples to oranges. They were really designed with different things in mind. But for the sake of comparison, they will not make as much power as the others mentioned. When the agenda is the most HP, these heads will become a restriction. They can be ported, but if you are buying new, you might as well get a better flowing head right out of the box. Price wise you can get Edlebrock or Trick Flow for close to the same $$$.
They're good heads if you can get a used pair for like $500-$600, otherwise for what they cost brand new you might as well get some AFR's or Trickflow's.