The PA sale now has this item made by karanta:
24 High Resolution Terrains for Bryce 5.5
Will this item work in Bryce 7 Pro?
Yup Most Bryce content is backwards compatible.
However I have to be honest and say that terrains actually made for Bryce 7 will probably be more detailed, as there are a lot more terrain tools in Br7 than there was in Br5.5
I have virtually every Bryce product made before Bryce 7 and all of it so far has worked flawlessly in Bryce 7. I do have to agree though that alot of that stuff is a bit dated and from a time when computers and programs couldn't do as much then as they do now. So in other words, those were hi resolution terrains by the standards at the time but are not as much so by todays standards. That being said Karanta does fine work and makes good products or did anyway, I have not seen anything new under the karanta name for some time now.
Bryce is generally downward compatible. Also tutorials that were made for Bryce 4 and upwards are still valuable.