Your best bet is to measure your gas flows. Natural gas = 1000 Btu/cf. Then look at a Sankey Diagram for a furnace. Energy In = Energy Out. IN is the fuel gas, OUT is hot flue gasses, heat into product, and losses. Kiln losses typically are from shell and from holes/leaks. Be cautious...
Adding insulation is typically your best bet. Changing pipe size can have unintended consequence as others have pointed out. Reminds me of the guy who piped his house water at 3/4" to save pressure... had to wait (0.75 / 0.5)^2 times longer for his shower to get hot.
Main point I wanted to...