I'm not following your reasoning on the D-Cannon being great against non-Titans. Three Scorpions average just over 3 kills against Russes, while 6 D-Cannon hits do the same. However, the difference in range between a move & shoot D-Cannon and a Scorpion Pulsar is enormous - 55 vs 85. With Scorps, I'm much more likely to be able to hit the enemy with a single move and shoot and I'm much less vulnerable to retun fire.
If I were going to take Cobras, I'd take pairs, since I don't think that the extra blast marker is worth 250 points. A pair of Cobras are slightly better than a pair of Scorpions against 4+RA, assuming 6 hits (which you'll really only see against Guard, and I've been completely unable to figure out how to get my regular Guard opponent to sit his Russes within a move of my Cobras), but I think you'll find that most people far prefer the range and versatility of the Scorpions. Not only can they hit from much farther away, but they're also far superior against anything without RA (infantry in cover, Tau vehicles, other Eldar, Predators).
At TK(1), I've got absolutely no reason to take them over Scorpions. Even against a Warlord, my pair of Cobras are only going to get 4 attacks, of which 2 ought to hit. My Scorpions average 4.2 hits and 2.1 failed saves. At TK(2), they're at least better than the Scorpions at taking out Titans in ideal circumstances, and that's close enough to their current state that I can't tell you whether it's too much of a nerf or not. I have a feeling that you wouldn't see them, though. MW3+ TK(D3+1) is just a much weaker Volcano Cannon if I understand what you mean correctly. It's less accurate, much shorter ranged, is mounted on a much more expensive platform, and makes up for it by doing very slightly more damage to War Engines when it hits.
As well, it's not like they're the only things that can do that to Titans. Deathstrikes are certainly quite capable of accomplishing much the same thing, and trios of Shadowswords certainly aren't lacking. A pair of Cobras gets 4 attacks, 2 hits, and an average of TK(3) against a Warlord - that's 6 DC in damage. A Deathstrike volley is almost identical (5.83). 3 Shadowswords average 5 damage for the same price, and they do it from much farther away. The Shadowswords also average 5 damage against War Engines of any size, whereas those Cobras only average 4.5 against DC6 and 3 against DC3.
Though not quite the direction you were going in, one change that I'd consider for the Cobra is to remove its blast template, making it much less effective against most things, while pumping up its TK value even more to compensate. Something like MW2+ TK(4) might work. You get twice as much output against War Engines as a Volcano Cannon, while being just as good against most things. I have no idea whether it'd be worth more or less than 250 points with this, though.
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