I've run that harlequin squad a few times, and never been really thrilled with the results. The fusion pistols are nice, but most of the time it turns out I need to run to make it into assault. At that point, they're just S4-rending instead of S3 power weapons. If they weren't like 22 points per model, they'd at least be putting more hits each in, but fully upgraded 10 banshees cost 192, versus the 182 for six harlequins. The only significant difference in practice was that harlequins didn't get heavy boltered to death.
I do think three WL's may be the way to go though. Here's revision 3 - the plan is for the guardian squad with the warlock to sit back on an objective. The lance-toting wraithlord will find a big tree to hide behind (hopefully) and babysit them. The warp spiders will either slide up a flank or arrive by deep strike, depending on the scenario. Everything else will form a 1500 point bunker of doom and trudge across the field in relative formation.