It seems that you should lose the AC from Tumble for that, since the Armor Check Penalty represents the restrictiveness of the armor and shield and the Tumble skill's bonus to AC represents being more difficult to hit because of fancy acrobatics. You can't really be a solid bastion behind a tower shield and a nimble, flipping acrobat at the same time.Who knows what Bioware did, though...
Skill focus wont help you I believe, it is simply pure base ranks, no bonus of any kind is added to them when they calculate the armor bonus
From LORE:For every 5 ranks in this skill (base ranks only, not including feat-, dxterity- or item-bonuses) the character
So at high levels you have two warriors facing off against each other, one has spent his life smashing anything that comes in his way, the other has spent his life perfecting working his skills in line with his defensive capabilities, working out how he can move and use his shield to best aid his body protection, how to move his feet or shift his hips to deflect the angle of a blow or to help make a blow more effective or his parry better..