We don't breed, but I certainly wouldn't breed a HH horse. I might consider a NH one, though (but I haven't really thought about it), but I wouldn't do it to attempt to get an Impressive-like halter horse. There are plenty of big muscled NN horses off Impressive, too. Our 'Lady' is off Impressive, is NN, and is very stocky/large muscled.
One thing I don't like, though, is that many people think that all Impressive horses are 'defective'. I think we need to be careful..there are plenty of other known genetic defects and certainly many more that are not known and/or haven't been publicized.