In an attempt to not sound too authoratative (my physics has been many years past and I do software at NASA not physics)… as stated in the previously mentioned thread… something like “putting the myth to rest”… Rich is right about the displacement/boyancy/float… but there is another factor that the comes to play. While the boyancy/float factor of displacement/weight plays a small roll in proportionally speaking the weight difference is minimal and the relationship is essentially linear… The rate the lighter object accellerates to the surface is an expendential relationship with respect to its change in weight/displacement relationship.
What does all this theory mean in the real world… a slightly lighter board (eps vs pu) will float about the same meaning little to no difference when paddeling (except in how chop defelcts the board), but when riding the board and duck diving the board will want to pop or return to the surface more quickly. This is part of what gives the corky feeling. When you go to set a rail it will tend not to stay set as deep or as long. Perimiter rails/stringers will help with this a little as the weight will be out at the edges/rails helping to sink/set a rail.
I'm also happy you mentioned something about Resin Research's flex standard. I have not read anything about that on this site before now.
as for the flex of different epoxies… several of the balsa composit sandwich threads have talked about this. Greg has 4 different flexes available in Resin Research epoxy. 1980, 2000, 2020, 2040. 1980 the stiffest and 2040 the most flexible. 2000 and 2020 being the more common with 2000 being the baseline standard RR epoxy. DanB’s “9 foot 12 pound” thread is one that talks about the use of different resins flexes.