I'd stay with Imp. TC.
The AE threat is invaluable, and you'll lose quiet a bit of damage mitigation by going Prot at that point (yes, irony) by dropping the 5% Parry and TC slow.
EDIT: Shield Slam is great, but it only starts being superb when you can stack +block value. While leveling it's likely to be a minimal boost over Sunder/Revenge.
Edited, Sep 22nd 2007 8:34pm by RPZip