Not only was there a burst of buying just before it went into effect, there was an appreciable slow-down in buying after. It was widely held that the increase was a strong factor in the recession worsening for a period after it went up. (For instance, this story recounts it this way: "Japan’s sales tax was introduced in 1989 and raised to 5 percent in 1997. The increase pushed the nation into a 20-month recession and caused then Prime Minister Ryutaro Hashimoto’s Liberal Democratic Party to lose a majority in the lower house of parliament for the first time." That accords pretty much with my memory of the event.)
If going from 3% to 5% caused that much pain, I can only imagine what going from 5% to 15% would.
The people that believe the sales tax going from 5% to 15% is a good idea are the same people that believe the trace gas CO2 going from 0.035% to 0.045% of the atmosphere will cause runaway global warming and destroy Earth.