I honestly don't see that much of a difference between memories and OP Strength in this instance. We know from Nynaeve's healing that there is a physical conduit to the OP in a Channeler, who's to say the 'Finns can enlarge or contract that conduit, perhaps they even have some kind of ter'angreal that does it

In the end we really have very little idea of what the 'Finns are capable of or are not. We know they have a stash of objects of the Power, we know they have the means to implant memories into someones head (likely some kind of telepathic ability inherent in the race). We know they are likely able to see events in the main dimension, IMO probably based on this telepathy. We know they have the means to capture and hold Channelers effectively (unless you think Lanfear and Moiraine were both burned out). We know they can read the Pattern in some way as to make very accurate predictions of Future events.

RJs put a few limits on them, like no transmigration, but until we see what happens I'm not counting them out as a possible reason Lanfear is stronger than Cyndane.