thanks, generally good paper to get a foot into what's, how, why fragmentation is happening. i posted my post in the ipsec section because i don't think its related to 2.0, but would be interesting anyway.. did you experienced problems with the same 1.2.3 setup on 2.0?
to me just setting the mtu on wan interface didn't solved my overhead problem.
you do site to site with psk, i think, there is more overhead depending on your configuration, lets say ipsec with rsa keys and x-auth has more overhead then, for site to site sufficient psk.
but as you can read on the other post, i`m by far not an network engineer and have hard times imagine myself what exactly happening in that profoundness.
if you're interested, jim advised to do mss clamping on vpn traffic, mentioned in the other post.
http://forum.pfsense.org/index.php/topic,29105.msg151281