This pepsis discussion ..... I will add, and I may be wrong, but the game does alot of buff/debuff snapshots. Therefore, like with warrior, I would/could actually line up my mitigation(s), wait for the incoming damage animation and hit shake it off to snapshot the mitigation buff(s) as it falls off. This if course buffs the shield of shake.

With pepsis I think, and I have tried this, used it as the damage (snapshot/animation) went off triggering the heals as the shield fell off. I'll need to record the process to see if the heals truly proc (^_^). If so, it's kinda an interesting skill play (latency be damned though). Depends on the response and priority of pepsis with the server as well.

Hence, you should technically get both benefits (damage reduction/heal). Again, I have not done a formal check. If not, than yes this ability is to "clear" your shields to, in some cases, add a bigger buffed shield or to get the mobility (emergency tactics) healing of E-prgnosis.

*Stronger shields overwrite weaker ones as many probably know*

Pepsis is also well suited to testing given that the tool tip says if E-prgnosis or E-diagnosis is not on a player they do not receive healing.
I'm not logged in so I won't be able to check this for a bit.