I've used square-taper and external bearing cranks on the same frame (80's Shimano 105 to new SRAM Rival GXP), and I can tell you that the difference in stiffness between them is enormous. What does that mean? I dunno. The only thing I can say about it is that the new stuff is stiffer. And, having used the 105 cranks on an 80's steel frame first, the crank/BB change was more noticeable than the frame with regard to stiffness. As for Octalink, it might be stiffer, but not that much stiffer than square taper cranks, and even if it is, it's hard to believe that it really matters. It's not an upgrade worth making unless you're looking for other benefits as well (better shifting, lighter weight, different gears). If you're not racing this bike anymore, why bother? The current set up works extremely well, so why mess with it?