Can answer my own question: I'd forgotten about the wonderful SpectrumViz, which gives us the key to cut through branding, and find out which papers are effectively identical (Thank you once again, Ernst!)
It seems closest ones to Canon 300 in RC satin paper are:
Epson Premium Luster / Semimatte (duh!)
and not identical, but v close: Red River Ultra Pro Satin (or Palo Duro)
basically these papers seem to be made by the same plant/ to the same formula
I had an old roll of Epson SemiMatte 250 (not 260) here, and that was ok, but definitely not as good as the Canon 300 in color/ depth. But it was the old 250 paper weight/formula, and clearly that was improved upon a while back.
Will try the RedRiver - interested in seeing how that does.