I'd like to chime in here and say that I feel a Jetbike Council is ridiculously expensive for an anti-tank unit. First, you have the Farseer himself, which will run you at least 120 pts in most cases. Then you add in the Warlocks, a minimum of 45 pts each, needing at LEAST 3 to even call it a Council... so that's another 135 pts, 255 pts so far and you have no powers (which you need) on the Warlocks, and no Singing Spears (yet). Now at this point you could take your "cheap" council, through some spears in it, and try to make back those 255+ points before your mini-council gets wiped out. Not too good a prospect.
Option 2: You go with a bigger council, perhaps a Fortuneseer & 7 Warlocks, with Embolden and probably Enhance. If you're shooting at tanks with Spears, you'll probably want about 4 of them, just to be sure... so lets say you put them on 4 of your warlocks (2 of which have the powers assigned to them), leaving you 3 ablative wounds in case you start taking damage. Now you are looking at a unit that costs over 460 pts. Though its a powerful unit with a lot of resilience, it will have a difficult time making back its points in an anti-tank role, unless you are fortunate enough to take out two Land Raiders (or equivalent) or 3+ less expensive vehicles.
What I am trying to get at here is that a Jetbike Council really belongs in Close Combat, where it can take advantage of its strengths (Fortuned Invulnerable Saves, Weapons that always wound on 2+, Enhanced WS, etc.) and minimize its weaknesses (low model count, large footprint that makes the unit difficult to hide from enemy fire, etc.). If you ARE going to put the unit in close combat, then you really don't want to spend the 3 pts for the Singing Spear upgrade, as you won't be shooting that much and you'll be penalizing yourself in hand to hand combat by reducing the number of attacks you get.
If you ARE going to use Singing Spears, I believe they should be put in very specific places. First, a solitary Jetbike Farseer without Warlocks could be kept cheap enough to do the job reasonably well, and since you'd want a lone character like that to avoid combat anyway, shooting would be an appropriate role for him. Second, a Warlock in a Storm Guardian Squad with Fusion Guns would be a good place (probably the better of the two actually) for a Singing Spear, as he would enhance that squads ability to conduct anti-tank operations. Personally, I would prefer to give that Warlock Destructor to diversify the unit, in the same way that I prefer to give a Fire Dragon Exarch a Dragon Breath Flamer, but that is a personal preference.
I hope you found this post insightful. I am not trying to discourage you, but hoping to point out some things that will help you make the decision that will work best for you. Good Luck.