Yes of course you are right that conversion rates won't be the same as purity because the carbon is removed as a solid. So the purity will be less than the conversion rate, as the left over methane is mixed with a smaller mass of hydrogen - one quarter of the original mass of the converted methane.
If you started with 100 kg of methane and a 97% conversion rate you would be left with 3 kg of methane, 24.25 kg of hydrogen and 72.75 kg of Carbon.
Purity of the gas would be = 24.25 kg / (24.25 kg + 3 kg) = 89% purity. Does that seem right to you?
Evidently even more work to do before you could put that through a PEM Fuel Cell which need over 99.9% purity.
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