Without the math I can tell you right now that in the current game, Penance and Flash Heal won't overcome shield spamming for throughput, doesn't matter how you gem. You could in theory, but the numbers required are unattainable. Without Divine Aegis, a crit Flash Heal alone, even if it crits for it's highest amount every time, is still less than 1 Power Word: Shield with 1 non-crit heal from the Glyph of Power Word: Shield. A non crit Flash Heal doesn't come close. If you add in the amount of the crit that would convert to Divine Aegis you have something to work with, but Divine Aegis will still cap at 10k so your target would have to constantly be taking damage for you to fairly count each crit Flash Heal for the full value (the full value being with the Divine Aegis attached.) But in this scenario you still need to be critting with almost every heal you cast, and you'd probably need to be casting 1 second Flash Heals without Borrowed Time to boot. Even if you incorporate Penance at every CD, Renew, Grace, and a single Shield on every Weakened Soul CD, you'll still be behind shield spam most of the time.

Someone has probably already done the math on this, between PlusHeal.com and ElitistJerks.com. If not it wouldn't be too hard to draw up. Maybe Zusterke might want to do it for fun after he reads this post. =P

As for straight shield spamming, it really depends on where you are in the game. With the 30% buff in ICC it's true that you no longer need to shield spam, but it used to be that's what you did. H Putricide at 0-5% is the most notable fight that seemed to require a disc priest shielding from the beginning of p2 to the end, never stopping except for an occasional spot heal. This was just because there was so much danger of players randomly (or not so randomly) being hit by something that could 1-shot them. Blood Queen and Rotface were also good fights for spamming from the beginning to the end of the fight. Princes you can play preemptively and Festergut you're full on the raid for half the fight.