Finally, some detailled info on this! Thanks so much, Dedude. This'll help me to adjust Scutumus before the free RCR disappears. Three questions that would help even further:
1) I've read your comments here and the wiki entry you linked, and still wonder about the following passage from the wiki:
The additional attack penalty while Shield Bashing mode is on is: 0 Light, 2 Heavy, 2 Tower. This excludes the default 'to hit' attack penalty for having a tower shield equipped, which adds another 2 to the penalty. The Agile Shield Fighter and Two-Weapon Fighting feats can each negate up 2 from the penalty.
Does this mean that if one uses a Tower Shield and has both Agile Shield Fighter and TWF, there is no penalty anymore to equipping the tower shield?
2) Does Northlander Hewing affect the Shield Bash in any way (with regard to damage, AB penalty, and/or attack reduction)? If yes, how? If not, is it likely to be modified in the future?
3) Does Weapon Finesse affect Shield Bash? I.e. is it or will it be possible to build a useful shield fighter with a sneak build (which is suggested by the TWF/ITWF/GTWF bash line)?