Well, two things.
First, water chemistry is extremely complicated, pH alone doesn't tell you enough to answer your question. How buffered your water is will also tell you how difficult it will be to change the ph.
Me, for example, I also have pH 8.5 or so water (Minnesota limestone baby!), and its crazy buffered. Just for fun I tried adding straight hydrochloric acid to a tank of water (no fish, of course), and it took a TON to get the pH down. And, a water change would have it pop right back up again.
In general though I agree with the above, it's extremely difficult and risky to maintain such a large change. Water changes would become a nightmare, and you'd inevitably screw up and the pH would swing out of control, potentially killing your fish.
I'm guessing you are looking at sensitive Amazon fish, perhaps Discus or something. If you want to keep fish happy in their pH, look at African fish instead, most of those lakes and rivers are closer to your tap water.