Dont forget that the super off peak single was reduced by 50% a couple of months ago. FGW▸ could if they want increase it by 100% as that would just bring it back to the same level as before.
The Super Off Peak fares didn't even exist on FGW before September. They are now the regulated fares on many flows so it's gonna be some loophole that allows them to increase the price. If the loophole is there, you can bet the shysters in FGW's fares dept will have found it.