I wouldn't rely on the holey rock to raise your PH. Over time it will help to buffer it but with water changes I don't think you notice it staying up at 8 by any means. JMO. I may be wrong as we don't have that up by us.
It does help, but there are more factors at work. It really depends on the chemical makeup of the water in your area. Some water is easily buffered some water takes a lot. I can not begin to tell you what the factors are because I don't understand it all.
For our purposes it is enough to know how different water will act different with limestone. Limestone is alkaline by nature. If you put it in acidic water (6.8 or so?) it will change the PH more and somewhat quicker than if you put it in more alkaline water like 7.8. Either way I find the change is pretty slow and maybe not worth the effort. Before the PH is changed , the rock has to partially degrade. That's a pretty slow process to me. If you grind that limestone up into cement and then put the cement in the tank it is real quick. The big lump of limestone has been setting around out in the weather for a lot longer than you want to wait and it is just starting to come apart. The first week or so of having it in the water may change your PH big time as it is covered in dust but after that it may take years. Like maybe 2 million.