It's better than just spamming Flash Heal. You'd need to rely on crit for them to have equal hpm, and PW:S doesn't overheal if it gets fully used, where there's every chance that Flash crits will just be overhealing.
[edit] The average heal of FH would need to be ~429, so you'd need around 31% crit.
=Healing=
===Penance===
*52 mana - 145 hps for 3s
*145 hps
*8.37 hpm
===PW: S===
*115 mana - 470h/15s
*31.33 hps
*4.09 hpm
===Flash Heal===
*105 mana - 327h/1.5s
*218 hps
*3.11 hpm
===Heal===
*33 mana - 163h/3s
*54.33 hps
*4.94 hpm
===Renew===
*57 mana - 59x4h/15s
*19.66 hps
*4.14 hpm
=Damage=
===Penance===
*52 mana - 86 dps for 3s
*86 dps
*4.96 dpm
===Holy Fire===
*41 mana - 183 + 42 (over 7s) 2s cast 10s CD
*112.5 dps
*5.49 dpm
===Smite===
*56 mana - 137d/2.5s
*54.8 dps
*2.45 dpm
===Mind Blast===
*64 mana - 197d/1.5s 8s CD
*131.33 dps
*3.08 dpm
===SW: P===
*73 mana - 174d/18s
*9.66 dps
*2.38 dpm