WowAce

I have created a repository cloned from r538 that implements the enhancement in a rough manner.
http://www.wowace.com/addons/xperl/repositories/hp2_r538a/

This clone of X-Perl contains a modified XPerl_Highlight.lua that removes the shield absorbption calculations and treats Power Word: Shield like a HoT.  The purple shieldBar now counts down from 15 seconds (max duration of Power Word: Shield) instead of trying to track how much shield absorption is left.

Reasons:
1) Patch 4.1 reduced the Power Word: Shield duration to 15 seconds which is identical to the Weakened Soul debuff duration (also 15 seconds).  For a discipline priest, the most useful information is not how much absorption of an existing Power Word: Shield remains, but the remaining duration of the Weakened Soul debuff remaining.
2) By having all of the Power Word: Shield durations visible on raid frames as a declining timer, it allows priests to plan ahead for reshielding a raid member (e.g. to use the borrowed time haste buff for a follow up prayer of healing or other spell).
3) By removing the PLAYER filter on the Weakened Soul debuff, it should allow multiple priests to see the timer left on each other's shields.

Hopefully in the future, I can make the two styles of shieldBar coexist with a menu option to select between the two.

If you finished it and make sure it's working fine, then port it to mainline by yourself please.

But if not, sorry I can't check with myself because some reasons.

This functionality is very specific to discipline priests. Until I can add a toggle option to allow both functions to coexist, this change would probably upset holy paladins who may want to track the amount of illuminated healing they've built on their targets.

It almost 3+ months and no more update or something so just close this old ticket, I guess they already fixed or can't fix or whatever.

Anyway, If you have problem with current X-Perl alpha/release versions, feel free to report them again. :D