MicroATX boards aren't just limited to the low-end like they were 10 years ago. There are still low-end ones, of course, but the upper-end ones are generally every bit as good as their full-size counterparts... just smaller.
The only real difference between the P5x and H5x is the support for the Clarkdale IGP. They should clock about the same.
Though, one thing to note; H55 is actually a bit deceptively named... the H57 is the one equal to the P55, H55 is slightly lower-end. The main difference is that the H55 lacks RAID functionality, and I think it also has a couple fewer PCIe lanes and a couple fewer USB ports (neither of which really matters much, IMO).