[c-nsp] MTU issue on a GRE tunnel

[ML]

I setup a GRE tunnel between two locations served by a Comcast business
class cable modem.

Config example. The remote end is the same.

Tunnel73
 ip address yy.yy.yy.yy 255.255.255.252
 ip mtu 1476
 ip tcp adjust-mss 1460
 tunnel source xxxxx
 tunnel destination zzzzzzzzz
 tunnel path-mtu-discovery


The two tunnel endpoints are ME3400s.  I expected that this
configuration would reduce the quantity of fragmentations needed by the
router but looks like "IP Input" was up to around 20%.  The user
experience was bad since most web pages would fail to load.  I thought
"ip tcp adjust-mss" would signal help signal the users OSs to act
appropriately given the situation.

Have I missed something obvious?