Taking slashes as a shadow priest hardly affects DPS at all. There is about a 6 second window, once every slash rotation that you cannot use SWD. As for pushback, with proper spell timing (and an eye on the slash timer) you can pretty much avoid taking any pushback at all by refreshing your instants during the slash cast. We're actually discussing this right now in the shadow priest thread.
TBC Shadow Priest 101: How to Melt Faces Effectively
Frankly I think it's silly to remove your shadow priests from the soaker spot because A) it cuts back on the effective healing of VE, which is huge on this fight and B/C) you're placing someone else in the soaker group who will require extra healing that isn't passive like VE
or you put no one in the group and run into potential situations where you have to swap in people to the soaker group because of burns. In all of those cases, the slash healing burden is simply being shifted from passive VE return to an active or reactive role for very little gain (very marginal shadow priest DPS improvement) or even a loss and potential wipe (more healing required on burns, more movement from healers or melee dps to cover slashes).