Give it a shot, if it doesn't work the you will only need to change it. You will not have lost anything, only gained the knowledge if it works or not. If your PH is falling, how far and their may be a problem with the KH level of your water. DO you have a KH test? Maybe you could pick one up. I use the Seachem KH/GH kit. Sometimes trial and error is the key to finding what works best. Anyone can tell you what worked for them but it may not work for you. If the water in your area suffers then maybe the lfs was onto something. The adviced I gave above may be out of place if you need that high of buffering, although unusual. I hope I did not confuse you, if so I appologize.
A bag of cc placed iny our filter may be another way to help your ph problem, this seems to be the way most go when a buffering problem occurs.
Brian