SwP should be refreshed if you recast once you have a full set of buff/debuffs, there is one notable bug as far as this is concerned is that you can on occasion get a 'more powerful spell' error where it won't let you overwrite the spell because it thinks a more powerful version is already applied. This is usually caused by a totem or crit enhancement buff being present when you try to recast since there is now a crit modifier applied to the spell as Spell Damage.
A more worrying dps loss is actually a bug with Mind Flay, which is of course our bread-and-butter channel spell.
Mind-Flay - 3 second channeled spell causing Shadow Damage
Since the last patch it appears that the damage ticks to do not all occur during the visual 'cast' of the spell. Essentially what happens is that you start channeling, the first damage tick will occur at some point (it appears to vary) during the first second, you should then have two further damage ticks. The problem is that due to the delay of the first damage tick, the last one could actually occur AFTER the channel has finished so if you chain your Mind-Flay you will be cutting out the last tick. This means that during a longer fight where an SP will be chaining MF between cooldowns of the other spells you could be clipping 10% of the potential dps or if you were just casting MF 33%!
Unfortunately cast bars such as Quartz can't help with this problem so the player has to time their recast after the previous one has completed by watching the cast animation and starting the new channel shortly after the animation completes. I recommend practice on a target dummy to get the feel for it by chain casting MF only and noting when the damage ticks occur and when they stop.