Not really the same situation.Wattage is a measure of clean power. You can’t draw blood from a stone.
Let's say that a correctly biased Champ with fresh filter caps and good tubes is capable of 4 watts clean.
Is the OP's amp putting out that 4 watts?
Also, that stock speaker is weak. The Jupiter speaker DOES get more volume out of whatever available watts the amp is putting out when compared to the stock speaker. It's a good "mod" if you're trying to get more volume out if a Champ.
Same deal with the 15 watt replacement OT that's available from a few vendors.
Same deal again with the 6L6.
8 clean watts out of a Champ is about as much as you can get. After that, you would need another amp.
Is it worth the trouble and expense to make the changes for an 8 watt Champ? Maybe, maybe not...
The OP's question was more like "can it be done?"