Exhaust valves leak

I just had a leakdown test performed and the #1,6 cylinders had 20% leakdown while the rest were >10%. They said it was most likely leaking from carbon buildup on the exhaust valve seats. Is there any way to clean this without rebuilding the engine? The engine has good power (dyno tested), compression is in spec, and it starts great. I was hoping to find some fuel additive or something else easy. Also, if I don't do anything to fix it what will happen down the road?

Thanks,
Chuck
'83 930 cabriolet